Jacqui Cheng at Ars Technica gives the most in-depth breakdown of Apple's iOS 4 I've found on the web today.

Ars reviews iOS 4: what's new, notable, and what needs work: "Despite its flaws, the iOS 4 update is certainly a solid one with enough new features to make it worth upgrading for pretty much everyone. Trying to live without even the most basic aspects of multitasking (notably state-saving) can be frustrating once you have used it for a while—during the time I wrote this, I continually switched back and forth between an iOS 4 phone and an iPhone OS 3.x phone, so I know from experience."

If you're upgrading it really is worth your read. I've certainly picked up a few good tips from the post.