So, I'm pretty much lovin' iOS4 in my 3GS. Multitasking is ok, but I don't swap between apps enough to really make a difference, and it'll be more useful when other apps can resume state.
App Folders are wonderful!
I haven't heard about it, but the Calendar gets some nice tweakage with regards to selecting which calendars you can see. I have several (Personal, Work, Medical, Birthdays, etc), and I most often live in the Monthly view. Instead of selecting all-or-none as in iOS3 (the name is retroactive, right?), and choosing to view all or each individually, you can now choose to show or hide each calendar. If you only want to see a particular cal, hide all and check the one you want to see. Go check it out.
I don't have much to complain about, however I have seen a drastic amount of emails fail to send immediately. Nothing else about my routine has changed, but I have a lot more warnings about messages going to my outbox for later delivery than I did before. This happens both on WiFi and 3G. Either there were no warnings for this in iOS3, the warning's timeout is too short (I have pretty good WiFi and 3G coverage), or there is some performance regression in iOS4. I can't say that I remember the Outbox messages one way or the other in iOS3, but I think the other items are likely. I've seen other performance issues elsewhere in the system exhibited in the form of slowdowns and stuttering, but they have resolved themselves pretty quickly.
All together, I'm very happy about iOS4, but I'll be quick to install 4.0.1 when it's released...