AT-ST

The AT-ST (along with its partner in crime, the X-1 Viper Droid) is one of those pieces that people expect to win the game for them. You look at its 40 damage and Double Attack and Damage Reduction and you think, "What could possibly stand in my way? I will not only defeat my opponent, but I'll also rob a convenience store with this monstrosity on the way home!" Well, I'll tell you what can stand in your way: +8 Attack and Mounted Weapon.

In and of itself, the AT-ST will probably lose against any other similarly priced piece in the game. Yoda, Darth Maul, the aforementioned Viper Droid, all of them will beat the snot out of an AT-ST more often then not, because all of them have real defenses (numeric in the former two cases, special ability for the Viper). If you can't hit your target, 40 damage doesn't do any good.
Fortunately for the AT-ST's playability, it's in an extremely nurturing, supportive faction that will love it for what it is inside (a non-Unique follower). The Empire's Commander Effects are second to none, and you can build a good squad around the AT-ST, but that's what you have to do, build a squad around it. Don't just throw one in and hope to win, because that sort of behavior is reserved for Aurra Sing (who the AT-ST needs to roll a 17 to hit in cover).

So, how do you support the AT-ST properly? The short version is Grand Admiral Thrawn and General Veers. Once you throw +3 to Attack and Defense, Force Immunity, and Accurate Shot on to it, everyone starts to worry about the chicken walker. If you're perverse, you can throw in a Stormtrooper Commander for a +10 Damage bonus. You can also use Imperial Officers for extra attacks. Weequay Thugs are in inexpensive source of Gunner for combining fire, plus they contain eight essential nutrients for growing bodies. You should also consider some Industrial Repair, since any squad built around one piece should be able to keep that piece in play as long as possible.

Oh, and the AT-ST also sports Grenades 20 and Mobile Attack, but these are practically footnotes. Mobile Attack is a great ability, but difficult to get much use out of on a huge, Rigid piece. Grenades 20 is really only good against a really large cluster of enemies, but at least it's there if you want it.

Speaking of Rigidity, the AT-ST can run into major problems on many maps. Be careful of allowing the battle to happen somewhere where the walker can't help.

In the final analysis, AT-STs can be terrifying, but only in the hands of a really strong player will they be truly effective. Damage output is only one element of this game (albeit an important one), and the odds that an unsupported AT-ST can hit anything you really want to put your two 40-damage attacks on are slim.

Overall rating in 100: 2 (you just can't afford to support it here)
Overall rating in 200: 4 (consider running two once you've bought your support pieces)