I have been wanting to switch off of TMobile for a long time due to the lack of high speed data they offer.
I was looking at Verizon, who I currently have my PC card with, and Cingular, who I am currently demoing the 8525 with.
I just looked up their data coverage, and found out that Cingular has a much smaller footprint on chicagoland when compared with Verizon.
Not the biggest issue, but I do have some clients out West where Cingular would drop me to their slow network.
Update: As far as I can tell, the Cingular 3G network is terrible. Browsing on it is no better than my 56k max connection on TMobile. Bandwidth tests show good results, but acutally trying to get websites to show up is terrible.