This is paramount, since coverage still varies widely in the U.S.- even from street to street in dense urban areas. YMMV. As you probably already know, a shiny new 3G phone does little good if the network isn't adequate where you use the service ;).
It would be helpful to know where you live/where you plan on using the phone the most. Even a zip code works. Will you be travelling to specific locations in the U.S. or abroad(?) GSM carriers are *generally* not as reliable in the western half of the U.S. once you are off the major highways and cities. For that reason you might choose a CDMA/EVDO carrier such as Verizon or US Cellular. If you are sticking to major metropolitan areas you have better options; a GSM carrier might be better if you plan on using the phone outside the U.S. Again, help us help you by providing more of your needs. Keep in mind some carriers, such as US Cellular, don't provide street level coverage so that might hinder you somewhat.
FWIW, another site for useful wireless info (plenty of reading, mind you) is howardforums.com
So I'm looking for a new phone to buy. The carrier doesn't matter to much but any info you have on carriers like if they have good service or anything that would be helpful would be great. I was looking at the LG Vu and it seems like it is a good phone but I have been reading a lot of mixed reviews so I don't know. I've already looked at the iPhone 3G but something about it just doesn't really appeal to me. So any suggestions on which phone is good would be really great thanks.