I had the LG Optimus G for AT&T for about 3 days, and it was a wonderful phone in some regards. The screen is gorgeous but does suffer from color banding. The phone is FAST, loads intensive games quickly and without a hiccup, and could switch quickly between all applications. The call quality is good enough, not quite up to par with my iPhone 4. The build quality was great, although kind of difficult to use one handed with a thin case on it.
The killer was battery life. It appears there's a critical bug draining it as noted on xda forums. Also, a small update AT&T sent out appeared to break the HDR and panorama function on my phone. The camera was not very good to begin with, as even my iPhone 4 outperformed it.
Customization was great and I really liked the LG UI. If the battery issue could've been resolved, I'd have kept it. But I can't take that risk when I only have a 14 day trial on AT&T. I ordered an iPhone 5 from my AT&T store today. Kind of upset when they said I'd be waiting over 3 weeks for it to come in.