Devilfish
A Tau APC designed to move Fire Warrior teams across the battlefield to prime firing locations to deliver the Killing Blow. A vital component of (now-defunct) Fish of Fury. It is fairly powerful vehicle on its own, possessing one of the best armor amongst dedicated transports (outclassed only by Wave Serpent and Ghost Ark), as well as respectable firepower of nose-mounted burst cannon, and SMS (or gun drones, though nobody use them), as well as up to two Seeker Missiles. It could be upgraded for night fighting (blacksun filter), dangerous terrain combat (sensor spines), or even city fights (PDTS and flechette discharger). The only things of Devilfish does not have are fire points and assault ramp, though, considering fire warriors usually have no special weapon, and 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 people transports are in the real world - Fire Warriors always take to battle in Devilfishes, unless the battle comes to them, and when a battle becomes too hot, the Devilfish drags them out of there again (because the Tau Empire have actual morals and want to keep their Warriors safe, don'tcha know). The Devilfish is dedicated each fire team, carrying ammunition, grenades and medical gear for the team, and having a powerful nose gatling gun to keep the enemy pinned while the team goes to combat. It even has 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.
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. In 5th, it was mainly known for the outrageous Fish of Fury maneuver, which had nothing to do with the strength of the Devilfish, but played on 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 AV are right about the most sturdy values for transports, and on top of that, it is a Skimmer, which only fuels it with more awesome than your average box. 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 S 5 AP 5 gatling pulse cannon, and carries two Gun Drones, who can dispatch 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 comes the upgrades.
The only two weapons that can be upgraded on the Devilfish are the Gun Drones, who can be switched out to become Smart Missile Systems, who are also S 5 AP 5, but ignores Cover and has a range of 30". This is not a hard choice... And you can give it two Seeker Missiles. Apart from that, most upgrades are small but vital - the Distruption Pods are in particular neat, since they give 5++, and the cheap-as-hell Sensor Spines lend Move through Cover for barely nothing. Others are less vital: the Automated Repair system is cool, but rarely goes into effect, the Fletchette Discharger and the Point Defence Targeting Relay are great for melee defence, but you shouldn't really be that close anyway. One upgrade you always should, though, is the Blacksun Filter. It's 1pt for ignoring Nightfinding. Might as well.
One advantage you don't see mentioned often is the size of the 'Fish. It is easily 1/3 lager 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.
Overall, the Devilfish is a solid transport that will do it's job very efficiently, but just not too point-efficiently. Regular Pulse Rifle Fire Warriors can do their job well enough without it most of the time, and Pathfinders need better hole up somewhere than move about. The only force that really can use it properly is Fire Warriors with Pulse Carbines and 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 propably 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 carry, but in expense, it costs a lot of resources that can't be used to kill enemies of the Greater Good. Should you use it? Fluff says yes, crunch says maybe.