I’ve been using CS4 now for a few days – I installed it (perhaps stupidly?) midway through a project, but so far it hasn’t screwed anything up majorly. It does a few things differently from CS3, and seems to have a few odd bugs, but presumably these’ll be ironed out. The best thing about it so far is a better Media Encoder, which works a lot more like After Effects, in that if you’re editing something with multiple sequences, you can send them all to the media encoder in a queue, and then go away and leave it – rather than doing them one by one with CS3, then being told when it’s almost done that “the application failed to return a video frame” when it runs out of memory. The Speech Search function is a bit of a white elephant, as far as I can tell… I used it on a talking heads clip with decent audio and no background noise, and it transcribed nothing but gibberish.
Oh, and another possibly useful thing is that formats are now set on a sequence rather than project basis, so you can have one sequence at SD res, another at HD, or whatever. Haven’t needed to use that yet but it sounds like a good idea (and one that all the other major editing packages have had for a while anyway).
I don’t think it does anything you can’t live without, compared to CS3, so if budget is an issue I don’t think you’ll lose anything by sticking with CS3 for a while. Plenty of people still use CS2 after all.