Yeah, framerate can sometimes slow but I have yet to experience any thing that are so bad that the game is unplayable. Titan Quest even on my current computer with decent cpu and gpu still hic-up on occasion and Dawn of War 2 is outright come to a crawl at times.
As for bug, there's one quest you can't finish because the dungeon you zone into just not there and that's the only quest bug so far that I run into (it's call Underground or something like that). They are aware and going to patch it in with the next patch. Sometimes the menu would glitch or you teleport to a place where you can't move but you can always go to the big Map and rezone anytimes.
I haven't have much problem with on-line play at all and I play on line all the times with my 2 kids (one is local co-op the other is over Live). Last night I group with people that are killing the big squid guy and there was not any slow down that I notice.
@JPL78, send me invite usually I am just mugging around doing the odd quests here and there. I recently change to Silver level because Bronze was getting really boring. My Seraphim just got her first tiger mount and she's mobile BFG turrent!!
