One thing to keep in mind is that even though the Hero is a great little Android phone, it is an older model (well, old as far as cellphones go at least)...and is not "cutting edge". It's no slouch, but it's not a speed demon either. I like mine, but I'm not what you would call a power-user. I just use it to make calls, send texts/IM's, twitter, facebook, pandora/sirius, and browse the internet.
If you want a much faster phone that can handle the latest-and-greatest android stuff, I'd suggest saving up and getting the HTC Evo that just came out.
Also, I heard somewhere that Sprint stores will give you a $50 trade-in credit if you give them your old Blackberry, but I can't verify that. Call your Sprint store and ask.