The books arrived on Monday and I had a chance to play 4e yesterday afternoon. I have to say that I enjoyed it very much. I thought the combat system was much more interesting than previous versions, the classes all had unique abilities, and the skill system is a more streamlined version of the d20 system. Play was a bit slow, but I expect that to increase with experience. Overall, I thought it was a big win.
The game is an evolution of the 3e/d20 game system. The core mechanic remains the same... roll a check against the appropriate skill/defense with a d20, but a lot of other things have changed. All attacks, defenses, and skill checks get a bonus equal to 1/2 your level (FRD). This makes save tables, BAB tables, and skill points unnecessary. Also, all combat actions now require a skill check, so your Magic Missile requires a Intelligence check vs the opponent's Reflex save. In practice, its a nice evolution of the d20 system.
I think the class abilities are what really make the game interesting. All classes have unique abilities, giving each class a well defined role (actually, a choice of two related roles). There are abilities than can be used once a round ("At Will"), once an encounter, and once a day, with the daily abilities being the most powerful. The At Will abilities are your basic attack, but often have other secondary affects, like push back the target, or give an ally a bonus, or force the target to attack you, and so on. What makes them interesting are the combinations that result, especially cross class combinations. It really makes the combat dynamic, where combatants move around to try to get advantage, or are knocked back by powerful attacks, or fire and retreat back to cover. And yes, the monsters all have similar abilities, making even low level monsters somewhat challenging.
Overall, I was very happy with the game. We only did a couple of combat encounters and a couple of role playing encounters, but that's normal when learning a new system. We haven't done a lot of exploring yet, so I don't have much experience with the new skill system (i.e., searching, opening locks, etc.). I'm playing in a couple of sessions at Origins. I'll let you know how it goes.