sourceninja hit it spot on. The biggest issue is IMs. Every time I receive a push notification (and that's not always, mind you, it does happen that stuff doesn't arrive), I have to launch the IM app (BeejiveIM in my case), it loads (1-2s), logs into their servers (depends, 1-5s) and only then do the messages arrive (another 1-2s delay). Meaning it's a royal pain in the ass. And is frustrating.
Luckily, I am Jailbreaking my phone. With Backgrounder this is a non issue, as the IM app runs fully in the background. Same for Twitter. Together with Reminder I get an icon on the status bar, when I get a new message. Battery penalty is really not noticeable (phone runs the whole day with plenty of % remaining when I decide to put it in the dock for the night). Could probably run 2 days without charging. The only problem I am encountering is that when in the IM app I am inside the chat window and switch to some other app (like the browser or a game). Since the app thinks I'm running it (as technically there shouldn't be any multitasking on the iPhone and also I'm directly inside the chat window so the message arrives and is not in "ready to view" or something), no badge or icon on the status bar. Easily avoidable though, I just go back to my friends list before switching to another app. A nice notification on the status bar and a red badge with the number or messages received shows up fine then.
This is a major flaw of the new iOS. It's not really multitasking and it's a really bad solution. It is an improvement for GPS and streaming music and VoIP. But this is just a workaround, a lame hack job. I will continue to Jailbreak my phone and run awesome apps like Backgrounder and Pro Switcher to have a great experience. As far as usability, this area on a stock iPhone is laughable.