Overwhelmingly positive. I've been involved in several ENWorld gamedays, and they've all been a ton of fun. We've got another one coming up in October. The ENWorld get-togethers at GenCon are also great -- it's like going home again (in a good way). Even the official games I've been in were pretty good, and considering it'st just a random bunch of strangers thrown together, you can't ask for much more.
From an organizational standpoint, location is usually the hardest thing to nail down. At the first one we had out at UMd there were probably 50+ ENWorld-affiliated people, plus a bunch of UMd students. Hard to find a place to accomodate that many and not have it cost you an arm and a leg. Our next one will be in my apartment's rec room. I really hope it works well, as it is reasonably priced and generally accessible, and should accomodate 30-40 people with no trouble.
The first one or two I just played. I DM more often than not when I have a regular group
so I really really liked getting to just play. Since then I've usually DM'd one session, though, as I felt some obligation to return the favor and let others play. There's definitely a different feel to DMing and playing in gameday games. Both parties need to just relax a little. The players need to trust the DM not to screw them over or make things needlessly hard. The DM needs to be reasonable in his expectations, and be able to accomodate different playstyles and personalities and skill levels.