There is a good article here: The Pilot's Lounge #23:<br>Crashworthiness — Improving Your Chances When You Have to Put It Down - AVweb Features Article
One of the points it made was:
"If you land gear up you hit on a fairly rigid structure that has only a few inches of crush distance before the load is directed to the seats themselves. The gear is designed to withstand and absorb a lot of impact load."
Anecdotal evidence at my previous employer from 3 off airport (Southwest US desert) landings in Bonanzas seems to bear this out. Two of the pilots decided to land with the wheels up, but both had back/disc injuries. Part of that was also due to the yaw caused just prior to impact by the passenger step hitting first. The guy that landed with gear down rolled over some very rough terrain, but the aircraft was rebuildable and he was uninjured. The two aircraft with wheels up were write-offs.

I'd heard of a study years ago by the USAF that recommended gear be put down during forced landings, but could never locate it. This article mentions it, though I don't know the veracity of the source:
Handling In-Flight Emergencies - Part 5 A Few More Things to Do Before the Emergency Landing - Flight Training
Here is the statement:
"Whatever you learn or decide about gear-up or gear-down landings, you can take solace in a study done many years ago by the USAF Flight Safety Research Branch. This study indicated that, in any sort of terrain, forced landings in tricycle-geared airplanes are less likely to result in injuries or fatalities if the landing gear is down."