Being a Mac developer, I actually got to have some hands-on experience with this new iCloud thing at WWDC. I must say it is pretty slick BUT it isn't without its flaws.
1. Not sure why they didn't make this available on their CDMA iPhone 4, but thats stupid.
2. It auto-syncs even when you are not connected to WiFi. In other words, you use your cell phone provider's data-plan without really knowing it.
3. So far the only use I have found is if you have multiple Apple devices. If you don't, its fairly pointless.
Honestly, I don't see myself using this, and I don't have every Apple gadget known to man (actually I don't have any Apple gadgets...only a single Macbook Pro). The biggest eye-popper for me was the $25/year 256kbps DRM-FREE (was shocked there was no DRM) music upgrade for my old CDs. Beyond that, nothing special unless your an Apple guru.