Tonight I played an Essentials class for the second time. The first was either a 1st or 2nd level Thief at a GameDay a while back. Tonight, I played a 4th level Knight. It was pretty good and as a defender, he did pretty darn well. The Knight is almost a combination of the Warden and the Fighter, and has a much more old school feel. In the end, did I have fun? Yes. Can it stand up to regular 4th ed Defenders in terms of effectiveness? I think so, at least at level 4. Would I want to play one in a long term campaign? Not at all. Honestly, the class is somewhat boring. You actually have a fair amount of options when making the character but when you're playing it, all you really have is a melee basic. It's modified a bit by the stance you're in but there is nothing terribly exciting about the class. There is a little bit of the 3.0/3.5 feel, where the combination of class choices and items can make for good synergies, but nothing to build a character around.
I remember feeling the same way about the Thief. That said, for new players and for players coming from older editions, they would be great. On the other hand, I think anyone would quickly get bored with the character, and if the rest of the party were composed of non-Essentials classes, I think you would feel outclassed.
All in all, the Essentials classes I've seen aren't nearly as bad as I thought they would be. But I still wouldn't want to play one long term.