I have a project I just finished that had around 2,000 clips (roughly 20 hours of raw footage) and it seemed to perform a lot better. Definitely started up quicker (went from taking 2 or 3 minutes to open the project down to about 15-25 seconds). Also, I seem to get much fewer "failure to save" reminders.
A lot faster... I have a project that used to take 3-4mins to load. Now that is done more in the background allowing you to get on with things.
Also - and this used to drive me mad - flipping between applications is a lot faster. So reading an email or editing a Photoshop image and then returning to PP doesn't take another 3-4mins for the reconcile. Much much better.
Stability wise... it seems fine. But haven't had the chance to test it that much TBH.
I'm working on a HD project with maybe 300+ clips. Everything seems quite "snappy". It's hard to know how much the improvement is due to CS4 versis my system upgrade from CS3/ XP Pro/ Intel Quad to CS4/ Vista 64/ 12 GB RAM/ Intel i7. I had to use a lot of workarounds on the old system for HD editing (no header thumbnails on the clips, etc.) to keep it stable for long projects, but so far seems good with CS4 & big RAM. I did have some stability problems with the old "QuickTracks for Premiere Pro" music software- which I think is probably obsolete for CS4/Vista 64. Switched to the latest version of the free standing "SonicFire Pro" & seems to be fine now.