I've been having problems with my Optimus V through Virgin Mobile.

It crashes in call (total power off); data availability hasn't been stable (I would say about once a week I have significant issues where I won't be able to use email, a browser, etc. for the entire day); maps/GPS is iffy; I think there are some interface design flaws.*

I used to have an iPhone 3G. I thought it was great.

I switched to VM for the plan -- $25/month for 300 minutes, unlimited text and data ($40/month for 1200 minutes).

Honestly, though, these problems haven't really bothered me too much. I've found that for $25 or $40 a month it's fine with me if the experience isn't "perfect".

*My friend also experiences these same problems