For vehicle to vehicle comms, GMRS really does work quite well. HAM offers broader spectrum of frequencies, higher power ratings, the use of repeaters, the ability to use the internet to ping location tracing, and if you are really into it, the ability to bounce signals halfway around the world off the ionosphere. But for day to day vehicle to vehicle coms, GMRS works just as well as Simplex HAM (ie, radio to radio, no repeater) .
The GMRS band plan is divided up into "channels" that are specific frequencies. There is a chart that shows you what maximum power you are allowed to transmit on with various channels. The "upper" channels are 15 to 24 and allow up to 50 watts for GMRS use.
Midland and Rugged Radio are probably the two most popular brands of vehicle mounted GMRS radios, with units ranging from 15 watts to 50 watts. The
50 watt Midland is a great radio, but it won't transmit on the lower power rated frequencies, so it is not a full range GMRS radio. The
15 watt unit however, will transmit on all the available GMRS channels, so keep that in mind. If you are with a club or group that prefers, say, channel 14, you are not going to be able to use that fancy expensive new 50 watt Radio.
I don't have any first hand experience with the Rugged Radio brand, but they appear to be more suited for side-by-sides (in my opinion). Midland has units that allows you to install the base unit under a seat or in the glove box and handle all the controls through the headset, which I really like. Midland also has a range of antenna styles with various degrees of gain / signal reception strength that are well suited for most vehicles.
A popular third option is to buy a variant of the ubiquitous
UV-5R HAM hand held radio from Baofeng, which (illegally) lets you transmit across HAM and GMRS frequencies, has fantastic battery life (with the extended battery) and multiple antenna options, and is generally quite easy to use. And dirt cheap. It is intended for use by people with a HAM license, and not intended for use on GMRS channels because that would be contrary to FCC rules.