I recently switched from AT&T to Verizon. The speed increase, at least in my area of rural Massachusetts, is dramatic. I don't think that 3G on AT&T exists in our area and so if I was able to get the BlackBerry Bold (or if I picked up an iPhone 3G) I don't think I'd see any difference over the anemic experience I had with AT&T. But with Verizon the speed changes the functionality of a BlackBerry. GMail on the BlackBerry actually becomes usable. This is an application that fetches at least the subjects of your GMail and keeps your mail all in sync (deletions on the mobile delete on GMail). On At&T GMail was usable only as a backup, if I needed to search for an e-mail and didn't have my computer handy. On Verizon, I'm actually able to use it as my primary e-mail application, ignoring the standard BlackBerry e-mail. Along with the ability of this application to also search my GMail contacts, which are now in sync with SalesForce AND the BlackBerry-Google calendar sync I'm pretty much set.
(Of course the thing that sucks about Verizon is that they disable GPS on BlackBerrys, but that's another story. Win some, lose some.)