Unfortunately, the game suffers from some serious balance issues. For instance, nothing short of metallic walls (and those are EXTREMELY rare) can prevent a character from being mowed down by a MG burst (pks notwithstanding), which makes burst specialists rather overpowered in the early to middle phases of the game. Also, the game is broken when it comes down to skill development as there are some items whose requirements you will NEVER meet (this is especially frustrating for engineers).
I made my lead character a stealth/melee/throwing specialist, but as soon as the "soldier" types in my party mastered burst fire, it all came down to bursting every enemy to hell. Just remember to bring plenty of ammo.
Nice engine, but the AI is pretty stupid, at least compared to what you can do. For instance, the AI will sometimes camp a position of advantage in front of some stairs, making your advance next to impossible due to suppression fire. However, a well placed grenade will make a huge hole in the floor that, in the unlikely event that the enemy survives, you can use to blast him to pieces comfortably. Just remember to bring plenty of grenades.
Also, the game pretty much gets f*cked up with the appearance of Pks, since most of your party becomes useless unless they can operate a Pk or a RPG. Snipers are basically useless (unless you get teh uber 1337 secret rifle), as are stealth characters and MG soldiers.
If only they had included a MP co-op feature... :'(
Be on the lookout for Hammer & Sickle, though, a RPG that uses the S2 engine. From the previews, it's looking pretty cool.