I have the Cingular card and as mentioned elsewhere, if you are in an area that offers the 3G service, which is many of the major cities, it is great - >1mbit downloads, ~200k uploads and really reliable.
If you aren't in a 3G area, you have Edge, which still gets like ~150k or so, which beats dialup. I imagine it won't be long for them to spread the 3G service.
Plus, you can use the card in Europe and elsewhere internationally, which you can't do with Sprint.
Spring does seem to have the most US coverage right now.