I average anywhere between 10-20 miles out with cingular before I got an antenna. I haven't been any farther then 20 mile since I got my antenna, but I went from only getting one bar on our standby buoy to 2 bars everywhere on the field. The other guys do about the same with nextal, alltel and verizon. Forget t-mobile if your going out of LA.
If your on Cingular extend while in the gulf your on petrocom and will be charged .80 a minute, which is better then the $3.00 it was until pretty recently. As long as your phone is GSM (cingular) it will work. If it's NExtal, Sprint, ext. forget it.