Check the coverage map for your carrier before buying anything. I have T-Mobile and there are large areas of the west where there's no coverage unless you're on an interstate highway. I looked at the coverage maps at T-Mobile, Verizon, and Cingular, and they all seem to have enormous coverage gaps in the west. I didn't have a cell phone the last time I was on the Colorado plateau, but my experience while traveling in general is that coverage is usually good along interstates and in or near decent sized towns. In remote areas it's spotty, and in the mountains it's bad on a good day.
Also, as noted by others, no booster of any kind is going to help if you're on the other side of a mountain from the nearest cell tower.