We ran encounters with 3, 4, and 5 PC's as the day went on. Started with a level one Warlord, Fighter, and
The 4e combat engine is a fine-tuned version of 3.0/3.5. People who are familiar with the latter will be able to pick up the new rules almost immediately. There are a handful of minor changes to be aware of, but the majority of these seem to be genuine simplifications/clarifications and not just meaningless tweaks. An example: what used to be called “being flat-footed” has been rolled along with some other things into “granting combat advantage,” but you no longer grant advantage during the first round of combat before you get to act. Attacks of Opportunity have been considerably simplified and no longer need to be tracked. The complicated rules for Grappling are gone, and as far as I've seen, you're no longer completely screwed if you happen to get grabbed by an opponent but didn't optimize your character for grappling beforehand.
From the DM's perspective, combat was pretty simple to run. The Monster Manual puts all of the info you need (including special attacks) right there in the creature's stat block. The ability to do this is a direct consquence of the streamlined combat engine. You still get suggested tactics for each monster, very helpful to those who don't idolize the great strategists of old (*points to my avatar*). The encounter-builiding section in the DMG seems like it will be immensely useful, especially for someone like myself who thought the old CR/EL system was next-to-worthless in practice.
For an Olde Skool dungeon crawl, 4e beats its previous editions hands-down.
Despite the simplifications to the overall engine, there is still a lot of temporary bookkeeping during combat. There are a lot of abilities with an effect lasting “until the end of your next turn” or “until the end of the target's next turn” that give modifiers to someone other than the person using it (these seem especially prevalent for Clerics and Warlords). Get used to tracking these things, as you'll be doing it most of the time. The flipside is that lingering effects (such as burning damage) no longer have a duration to keep track of: each affected target has a chance to remove such effects at the end of each turn with a very simple, typically unmodified roll (the new “saving throw,” though I wish they'd chosen a term without a previously established meaning).
Beginners will probably pick this up more quickly than previous editions, but there is still a steep learning curve. That's not inherently bad, but something to be aware of.
Disclaimer: This is largely a matter of personal preference. I haven't yet discovered anything inherently horrible about the mechanics themselves. That being said....
Guilty as charged, 4e does feel more like a CRPG/MMORPG than previous editions. Many abilities and mechanics make sense from the metagame standpoint but are completely artificial from the character's point of view (milestones being the number one culprit thus far). Said differently, the interface between you and your character is more obvious, and that's partly the fault of switching the way powers and abilities are described and rendered. I'm sure it's also partly the fault of WotC's desire to introduce their online tools, but that's a rant for another day.
But I stand by a previous prediction: we're going to see lots of expanded material in the future, and that worries me because of past experience. WotC will eventually pull out the “Hey, players want more options” card, and the result will be new things (I'm now guessing primarily class powers and/or feats, but some new races and perhaps a few classes thrown in) that are noticeably more powerful than the old, and we'll get the power creep all over again. Eventually the glut of options gets trimmed down and we'll see 4.x, and the cycle (what I've called the Magic: the Gathering model) continues ad infinitum.
In summary, it's doubtful that I'll ever run a 4E game, because it doesn't seem to mesh with my style of DMing. There are plenty of changes I don't like, but the system does have its merits and would work very well for combat-heavy games, and especially for dungeon crawls. Perhaps that's what was intended.