Most GMRS users get the license (you do need a license to operate GMRS) for one of two reasons:
1 - To have a radio that provides longer direct transcieving range because of increaed trasmitting power.
TRUTH: Get a GMRS radio with a removable antenna. Range is 99% antenna and 1% power. The primary antenna factor is height. GMRS signal is mostly line-of-sight. A hybrid GMRS/FRS which has a fixed, non-removable antenna, will have about the same range as a regular FRS radio. If you want to increase range, get a GMRS radio with a removable antenna and attach the radio to an external antenna which maximizes height - some use rope to haul an antenna up into a tree.
2 - To access GMRS-only channels and thus avoid the chatter on FRS frequencies.
TRUTH: The extensive sales of hybrid GMRS/FRS radios along with the unlicensed (illegal) use of the GMRS-only channels difficult in "active" areas - such as Disney properties and the like. Luckily the radios have limited range and in less popular areas you should find inactive frequenceis. Consider getting a MURS radio instead.
3 - To access GMRS repeaters
TRUTH: Use of a GMRS repeater can dramatically increase your range, since the repeater antennas are usually placed for maximum range in the area. Its that line-of-sight thing again. First you'd need a GMRS radio that is repeater compatible - that broadcasts on one frequency and recieved another. Second, you'll need to get permission to use most GMRS repeaters, since they are privately owned.
Some useful links:
http://wireless.fcc.gov/uls/ - FCC Universal Licensing
http://www.provide.net/~prsg/ - Personal Radio Steering Group site
http://popupcamping.tripod.com/frs.html - FRS Radios and the Real World
http://www.ultimatefrs.com/ - Ultimate Reference Site for Family Radio, GMRS, MURS and PMR446 Radios
http://www.gmrsweb.com/gmrs.html - Popular wireless magazine GMRS site