When I bought OSX Lion I also migrated from the development environment Xcode3 to Xcode4.
I'm running on a MacBook from 2010 and I'm getting used to the news in spite of the many changes.
With Xcode4, Apple has integrated the code development environment with the IB (Interface Builder) and that is an important change. Newcomers to Xcode4 are going to appreciate having just one environment to keep track of. Coming from Xcode3 I was rather confused at first, but now I'm quite happy about it. The IB looks very different. It took some time to find the Objects view, a favorite of mine, because to see it you have to press an obscure button. The icons for the different Inspectors have changed and I'm still wondering what they are supposed to resemble. The project settings are now easier to find and they seem to be fewer.
Another important integration is that of the Source Control. I don't use it, so no comment on it.
For my purposes, Xcode4 is clearly much better than Xcode3, except for one thing: 4 is slower than 3. Sometimes I have to wait for it. This hardly ever happened with Xcode3. I may be looking in the menus or something and suddenly Xcode4 shows the wait-cursor and it can take up to 20 seconds before it returns. If this happens it's important to remain cool. Just wait, don't click! Of course this is annoying, but I trust it will be repaired sometime. Products always mature when they get older as we all know.
Building the target is quick. Provisioning has been taken over by Xcode4. It's quite easy now; almost completely automated.
Quick help is quicker and always present and doesn't obscure the code window. A bug in Xcode3, which annoyed me nearly every day is gone with Xcode4. The bug was that sometimes Xcode3 got lost in the mark-up of the code and would show black, red, green or blue letters in a strange mix. The fix was to save the file (if you had made changes), close it and re-open it.
A disadvantage with the IB integration is that the code window has been slimmed. This has to do with my layout preferences, and anyway it wouldn't show up on an iMac, which has many more pixels on its screen.
Archiving for submission to the App Store is the same. However one thing could have been done better, and I'm sure it will be fixed. It's that the Distribution provisioning has to be set manually. But it could be my own fault, I'm not sure.
Finally: I advise those still using Xcode3 to migrate. "Je ne regrette rien!"