Our current campaign should be ending right around the time 4E comes out (we'll be anywhere from 12th-16th level by then, at this rate). I'm pretty optimistic about 4E, but I also might wait to read reviews before I buy the new PH.
I like what I'm hearing about 4E so far; it sounds like they're trying to address some of the problems I've had with 3.x. But there's also *tons* of 3.x stuff I'd love to run: Arcana Evolved, Ptolus, Expedition to the Ruins of Greyhawk, Necromancer stuff, Age of Wyrms, Savage Tide, War of the Burning Sky -- and now the Pathfinder campaigns look great, too. Actually, I prefer to use my homebrew setting, but I hate prepping for 3E. Especially statblocks.
So, basically, if the 4E play experience (and especially the DM experience) is clearly superior, we'll switch, and hopefully I can go back to creating my own stuff and have that not be a total headache.
Our current group is pretty casual, if the other DM and I want to switch, we will, and no one else will care one way or the other.
Also, I haven't bought any 3.x books since the announcement, and won't until after 4E debuts. But I am still buying minis and dungeon tiles.