Simple Mobile, well, I've see many threads about them on here.
It's an extremely long process to get it somewhat working. The device won't work fully.
You won't be connected to Blackberry Internet Service so E-Mail, BBM, and every other application that goes through Blackberry Internet Service (BIS) will not work.
If you really want a cheap plan, and a blackberry, I recommend Virgin Mobile.
$35 for 300 minutes and unlimited everything else is a steal. Just make sure Sprint has good coverage in your area since they use the same towers.