Here in AZ Virgin is totally useless and so slow you can't even open a webpage or anything with speeds of about 27Kb. T-Mobile on 3G works pretty well with 3Mb speeds if you can still find an older 3G device. 4G isn't much good though since they don't have much coverage and newer devices like Samsung 4G phones can't get 3G.
I am in Fl and have good coverage by most major carriers and wifes new iphone 4s works at 4 g on virgin down here and $25 a month. Sprint/Virgin doesn't work well in North Carolina either, they use sprint for their back bone which has very few towers in North and South Carolina, they sent us a micro cell but that only works at our vacation home. AT&T also does not work well in NC.
Just because the wifes new iphone 4s connects at 4G doesn't mean the speed is good enough to use it for anything. I was getting 5kbps tonight on T-Mobile 4G with 4 bars but couldn't run any apps. If you run the speedtest.org app what kind of speeds are you getting?