There are several rental places just off the ferry dock in Cruz Bay, and all of them are similar... in both price and service. You'll read about personal preferences or problems one individual will have, but in the end there really doesn't seem to be much difference between them. Most offer Jeep Wranglers and Liberties, Suzuki Samaris and Vitaras, maybe a few other makes of 4-wheel-drives, and even some of the safari-type trucks (pick-ups with benches and a canopy in the back) if you have a large group. Jeeps and Samaris start around $50-$65/day.
As for driving on the left, its not really a big deal, and you get the hang of it quickly. There is very little traffic anywhere outside of Cruz Bay, and the scenery is spectacular! A beautiful view at the crest of almost every hill! You can drive the entire island in a fairly short period of time, and see many of the beaches in the South and East End that the taxis don't like to go to. Rent a jeep and do St. John on your own! Its a lot of fun!