It'll be interesting to see how many people jump ship from AT&T and how many people trade in various other Verizon phones for iPhones.
At the moment, AT&T and Verizon offer a Morton's Fork to the phone connoisseur: AT&T's network is fast, when it works properly, but has lots of holes, even around here (AT&T is notoriously bad in Palo Alto, for instance). Verizon's network may have somewhat better coverage, but it's as slow as EDGE for data.
Maybe it'll be better with LTE, but of course, the Verizon iPhone isn't going to do LTE (which is only fair - the first iPhone was EDGE). We'll have to wait for that for probably at least another year.
The big news is the word that the Verizon phone will include a WiFi hotspot feature. It'll be interesting to see whether AT&T will add this feature, and how they'll price it (it really should be added for free to their existing tethering feature).
What has been left unsaid so far is whether or not Facetime will be supported without a WiFi connection (that is, over CDMA data). There's a much better case to be made for restricting it on CDMA given the constricted bandwidth compared to HSUPA.
My prediction is that Verizon will offer Facetime without restriction, which will force/shame AT&T into dropping the restriction on Facetime, and AT&T will follow suit with the wifi hotspot for folks who have the tethering plan.