Well, we've played 7 sessions now with my Essentials Thief in a party of PHB PCs. Now at 3rd level, I haven't seen any significant problems. I'm a bit more min maxed than them since they are pretty much all new to 4e, and I think it's fairly clear that Essentials characters generally have the edge over PHB at least at low Heroic - this campaign will go from 1st to around 9th level BTW, I'm sure a game which (a) started at the Epic tier, (b) allowed all sources, and (c) mixed E & non-E would see the E-classes somewhat outclassed.
My experience is that in long days of 4-5 encounters my Thief is clearly superior, whereas last night we just had 2 fights, and in the second we could nova. In that situation the PHB PCs nova-ing with their Dailies somewhat outclassed me, the best I could do was Backstab then AP for a regular basic attack. I certainly didn't feel useless though - I was the first PC to act and I threw my Dagger of Distance around 15 squares, through a bunch of skeleto minions, twice, to do around 47 damage to the Purple Ghoul before he could act.
