I've been a Treo user since the Treo 90 and have owned all the Treos except the 700.  Recently (yesterday) I switched to T-Mobile having acquired a BlackBerry 8100 Pearl from Amazon and I am completely satisfied.  I switched carriers because my Sprint service in Tucson was poor at best & even-though they would fix it after spending 45 minutes on the phone (conservative estimate); the fix only lasted a week or two & then back to constantly dropped calls.