Devilfish
A Devilfish is a Tau armored personnel carrier designed to move Fire Warrior teams across the battlefield to prime firing locations, ideally to deliver the Killing Blow described in Tau strategy. A vital component of (now-defunct) Fish of Fury tactic, it is a fairly powerful vehicle on its own, possessing one of the best sets of armor values amongst dedicated transports (outclassed only by the Eldar Wave Serpent and the Necron Ghost Ark), as well as respectable firepower in the form of a nose-mounted burst cannon and a smart missile system (or gun drones as standard, though nobody uses them), as well as up to two seeker missiles. It can be upgraded for Night Fighting with a blacksun filter, modified to navigate dangerous terrain with sensor spines, or to better handle close-quarters city fights with a flechette discharger or a point defense targeting relay. The only things the Devilfish does not have are fire points and an assault ramp, though, considering Fire Warriors usually have no special weapons, and the only things they want to charge are enemy tanks, they mostly don't need either.
In the fluff, the Devilfish is used the same way armoured personnel transports are in the real world—Fire Warriors always take to battle in a Devilfish, unless the battle comes to them. The Tau's "Light Infantry" who are the ones who walk on foot with less vehicles and heavy gear (if you use the US's definition of light infantry), are either the Kroot or Pathfinder units. When a battle becomes too hot, the Devilfish drags them out of there again (because the Tau Empire has actual morals and wants to keep its Warriors safe, don'tcha know its expensive to train a new soldier!). On the other hand devilfish could also be used like APC, the Devilfish gets the fire warriors to the fight and the warriors fight on foot from there (in this situation for each on foot fire warrior squad you see there is a Devilfish off the board), with the Devilfish used to pull out quickly if things go bad without risking your ride back or way to advance forward in the fight. The Devilfish is dedicated to each fire team, carrying ammunition, grenades and medical gear for the team, and using its nose-mounted gun to keep the enemy pinned while the team goes into combat. It can even have additional gun drones to help gunnin' if the Warriors need it. So they're essentially hovering club-houses, only the team-members are carrying fuck-off powerful plasma-snipers.
Drone Harbinger
The Drone Harbinger was a variant of the Tau Empire's Devilfish which was armed with only twin Pulse Carbines. It appeared only in the Dawn of War video games.
Whilst weakly armed, the Harbinger actually had the advantage in that its internal structure served as a factory in the production of Gun Drones. These cheaply produced weapons of war are capable of slowly overwhelming an enemy in their massed firepower. Most of the space within the vehicle is taken up by production and storage facilities for its Drones, and the rear half of the Harbinger has been enlarged to account for this. As such, a Drone Harbinger cannot carry troops. However, the vehicle does feature a hatch on each side to allow the Tau crew inside to disembark if necessary. The rear of the harbinger features a pair of deployment hatches for the Drones it produces.
Once deployed, the Drones disperse and attack any enemies that come within range, or can be directed to attack a specific target. However, each Drone produced by a Drone Harbinger only has a limited supply of energy to act upon before it is depleted and the Drone deactivates.
A Drone Harbinger's primary sensors are located in an array under the tank's forward housing in the vehicle's nose. The armor of the vehicle is made from a dense nano-crystalline metal, named Fio'tak in the Tau Lexicon, that is light, malleable, strong and corrosive-resistant, reducing the weight of the vehicle whilst also providing overall excellent protection.
On the Tabletop
The Devilfish is 80 points, which means that it isn't used much, which is sad, really, because it is a really great transport. Unfortunately, in 5th Edition, it was mainly known, and beloved, for the outrageous Fish of Fury maneuver, which really had nothing to do with the strength of the Devilfish, but rather exploited the rules for Skimmer transports. In 6th and 7th Ed, the 'Fish is back where it belongs - as an expensive, but efficient answer to enemy fire threatening your poor Fire Warriors before they can set up sniping lanes.
What you get for this steep price is a machine that your Fire Warriors will love you for fielding: 12/11/10 for armor values are right about the most sturdy for transports, and on top of that, it is a Skimmer, which only fuels it with more awesome than your average bawks. As a transport, it has room for 12 guys, which is just about a regular Fire Warrior team without gun drones, and it is armed with a burst cannon, the ever-ubiquitous S5 AP5 gatling pulse cannon, and carries two gun drones which can detach from the vehicle if needed. (This is a good thing to do, since it will let them pick other targets if needed, and will divert fire away from the Devilfish). This is, all in all, pretty awesome. But then come the upgrades.
The only weapons that can be upgraded on the Devilfish are the drones, which can be swapped for a smart missile system, which is also S5 AP5, but ignores cover and has a range of 30". This is not a hard choice. You can also give it two seeker missiles. Apart from that, most upgrades are small but vital—the distruption pods are particularly neat, since they confer a 6+ Cover save in the open (stacking with any other sources of cover, including stealth from Night Fighting), and the cheap-as-hell sensor spines grant Move Through Cover for barely anything. Others are less vital: the automated repair system is cool, but rarely works. The flechette discharger and the point defence targeting relay are great for melee defence, but you shouldn't really be that close anyway. One upgrade you should always have, though, is the blacksun filter. It's 1 point for ignoring Night Fighting. Might as well.
One advantage you don't see mentioned often is the size of the Devilfish. It is easily 1/3 larger than a Rhino, which makes it easier to hide away your precious Warriors if things get too hot. It also makes it hard to Deep Strike, if that was your plan.
8th Edition
So, remember all that stuff up there? Yeah, its defunct now, seeing as vehicles don't get systems anymore. However, it still is a nice vehicle. It has an Impressive 13 wounds, a 3+ armor save, 12" of movement at max health, can take up to two seeker missiles (which are useful for squeezing extra wounds onto an enemy tank,) and has a Ballistic Score of 4+. As per usual, you can replace the gun drones with a Smart Missile System, and can hold your own against a good number of units, while also retaining the infamous Burst Cannon. Another change from previous editions is that drones make a difference when embarked inside of it. Specifically, the MB3 recon drone(You know, the really tall ugly one that confers the ability to ignore cover to whole squads?), which confers the Devilfish with the ability to ignore cover as well. Kinda pointless if you are taking the SMS, and your Devilfish should be transporting Fire Warriors anyway, not Pathfinders. Another neat gimmick (and possible reference to DoW) is that you can now carry up to 12 Infantry or Drones inside of it, but the MB3 doesn't count towards that 12 infantry/drone cap. The AP value of its weapons is now 0, but it makes up for this in pure Dakka. Additionally, the fly keyword not only will help you against all kinds of terrain, but basically allows you to nullify Tarpit tactics.
In Conclusion
Overall, the Devilfish is a solid transport that will do its job very efficiently, but just not too point-efficiently. Regular pulse rifle-wielding Fire Warriors can do their job well enough without it most of the time, and Pathfinders are better off holing up somewhere than moving about. The only people who could really use it properly is a bunch of Fire Warriors with pulse blasters and/or EMP grenades. They need to be close, and quick, and they're not so cheap that it's worth leaving them. If possible, you should probably try and get them out of there with the Devilfish.
So, conclusion: The Devilfish is like the fluff. It is made to care for the Warriors it carries, but in expense, it costs a lot of resources that can be used in other ways to eliminate enemies of the Greater Good. Should you use it? Fluff says yes, crunch leans towards yes as well.