I love the playable Geth characters. Especially the Engineer and Juggernaut. The GI is incredibly fun to play too (Piranha is my fav), but I always feel bad when using him because he's just too good for Gold. I tried him with the Scimitar once on Gold, and that felt about right. I would like the Trooper more too, but I don't like any setup on him. I don't like Flamer-less builds because I don't like playing completely without powers, and if I'm going to burn stuff with Flamer, I'd rather use a Vorcha and scurry around cackling maniacally while doing so.
Playing against Geth? On Gold, it's usually not so bad if you have any situational awareness. The stagger doesn't really bother me that much on Gold, my biggest complaint is that they seem to like to re-spawn right behind you more than other factions (especially Pyros, Hunters, and Rocket Troopers). I don't usually play Platinum, but having 3 active Primes at once is just too much. Keep those damn dirty toasters away from me.