$20/month for unlimited data at T-Mobile for the win!
As someone else said, Sprint offers unlimited data for $10/mo on the older 1xRTT data plan and from my experience (I was unlucky enough to have Cingular a while back) it's no slower than EDGE.
I now get unlimited data and some Sprint TV, which is actually quite usable, for $15/mo. For an extra $5/mo, I get unlimited use of TeleNav which is better than the $1000 GPS I bought in my last car since it can calculate the best route given the current traffic conditions.
So, $20 for T-Mo EDGE is nice, but does not get the win.