I've been using Sprint for almost 2 years now, and have to say that I've enjoyed their service much more than my previous provider (US Cellular – they LOVED to drop my calls!)
If you're planning on signing up for Sprint before 10/31/09, you're eligible to receive a $25 Visa debit card. If you're interested, leave a comment on this post and I'll email you with instructions for getting your $25. The process is pretty simple – after you get your new phone, you call up Sprint, tell them you were referred by an existing customer, and then they mail you a $25 Visa debit card.
EDIT: I should also add that if you've ALREADY switched to Sprint (within the last 15 days), Sprint's rules say you're eligible for the $25 bonus debit card thing.