I am with Cingular (original orange customer) and had a Motorola v600 that I truly grew to hate which is another story.
My wife had already upgraded to an Audiovox SMT5600 which she loves. To be honest, I went to the Cingular store looking for a 2125, but the sales person showed me the new 8125 (just released that day).
I was originally wowed by the 8125, but after 6 days, I returned it for a 2125. The main reason being that I wanted a 'phone-centric' device with some PDA/PIM features and not a PDA with phone features.
I wanted a one hand device (here I liked dialing and navigating with the 2125 better than the touchscreen or sliding keyboard of the 8125).
In addition, my 2125 has noticably better reception than the 8125 did (this is not necessarily indicative of the phones in general).
The 8125 has wifi, and the 2125 doesn't; but the 2125 with edge is fine for my needs. You should also compare the prices of the data plans for the 2 devices ;-)
So, for me the main decision point between the 2 devices should be if you want to use the device as a phone first or a PDA first.
Hope this helps.