I have Sprint and love it (I work at Radioshack and we have employee plans with Sprint, ATT, & T-Mobile and I like Sprint the best)
Speeds are great (especially in 4G). I have the HTC Hero with CyanogenMod v6 which is Android 2.2 (Froyo). I used to have the official 2.1 SenseUI installed.
It all really depends on where you live. If you're in an urban area, there's a high likelihood you can get 4G from Sprint in which case you get the EVO or the Epic and you'll be amazed! If you're in a rural area, it's not as fast, but still decent. Plus, they're way cheaper than ATT and Verizon especially when you are considering a data plan!