Sentinel
The Imperial Guard AT-ST Sentinel is one of the Imperial Guard's most recognisable vehicles. With its long legs that allow a superior field of view and fast movement through nearly any kind of terrain.
Patterns
There are two common patterns of Sentinel: the Scout Sentinel and the Armored Sentinel, both with comparable, but different goals and two Forge World variants, Drop Sentinel and Sentinel powerlifter
Scout Sentinel
The Scout Sentinel is outfitted with a number of devices and modifications to make it more adapt at scouting: gyro-stabilisers to navigate the most inhospitable of terrain, noise reducing modifications to avoid detection, and sensor and auspex systems to detect the enemy. It is rather vulnerable to small-arms fire due to its open crew compartment.
The Scout Sentinel can field a variety of weapons. Standard armament is the Multilaser, which can be replaced with a heavy flamer, an Autocannon, a Missile launcher, or a Lascannon. Earlier versions could also field an Assault Cannon, but due to munitions problems this design has been abandoned.
Armored Sentinel
Forgoing its stabilisers for recoil compensators and its detection systems for additional power cells and cooling systems, the Armored Sentinel can fire its weapons while charging at the enemy. Its superior armor compared to the Scout Sentinel allows it to shrug off small-arms fire, and protects its pilot even if hit with a heavy weapon. The Armored Sentinel can carry the same weapons as the Scout Sentinel, with addition of the Plasma Cannon. Still retaining some of the Scout's movement capabilities, the Armored Sentinel is perfect for flanking the enemy and hunting enemy vehicles.
Drop Sentinel
A.K.A the "Suprise Sentinel", drop sentinels are normal scout sentinels with Deep Strike and Multi-Melta. They come with heavy bolter as default but if you don't take meltas then you are doing it wrong. Drop sentinels also get Sky Talon as dedicated transport. Take that and rocket pods for 80 points valkyrie that does not take force organizarion slot. In-game use drop sentinels to deep strike/grav chute behind enemy artillery/heavy vehicles and melt them from behind
Sentinel Powerlifter
The 5th Edition ultimate troll machine. Take these things in squadron of 3 and throw them into TEQ/MEQ's for lols since they can tie them up for remainder of game, or buttrape said MEQ's with sentinel's powerclaw attacks that strike at S8 AP2 A2. After 6th edition hit, Powerlifters main tactic got nerfed to hell since most MEQ's now carry grenades which can blow them up in an instant.
Use on the Tabletop
Although cheap, the Sentinel suffers the obvious handicaps of a Guardsman's ballistic skill. As a gun platform, its hits are massively unreliable, even in squadrons of three, and there are more useful things you can buy for the price of three Sentinels. Scout Sentinels have the advantage of Outflank, but with their low armour, they will survive only as long as your opponent is gracious enough to ignore them. Factor in their BS, and they won't do much damage before bolters pick them off. In 5th edition, Armoured Sentinels made excellent tar traps, too weak to kill anything in close combat, too armoured to take any damage. You could tie up an entire squad of Chaos Terminators for an entire game with ease. 6th Edition allows players to choose to fail their leadership and retreat from close combat, nullifying this one beautiful piece of trolling (but you can still chase them off the table afterwards).
It should be noted that sentinel that is in cover at least knee high and has camo netting (which is a must for sentinels, if you don't take it for every one of your sentinels for every battle, the Emperor shall smite you down and laugh at your corpse) gets +3 cover save. Combine this with plasma cannons and heavy sentinels for 3 blasts at S7 AP2 and you have terminator/light vehicle/spehs mehreen melt squad. Or get Leman Russ Executioner for same price.
Really, wait for a new codex with cheaper and/or more powerful Sentinel variants and forgo these walking Star Wars references in favour of something a bit more shooty. The new codex has the cost of armoured sentinels down to just a few points more over scout sentinels, scout sentinels can no longer take plasma cannons, but all their weapons had point reductions to them. So a lascannon costs as much as a plasma cannon. With vendettas no long the default take in the fast attack slot, there is room and points to bring in this slot. Going from a fluff only choice for an army to something to consider to fill certain weapon gaps.