On road: wear the belt. I shoulda died, but I didn't. Instead I walked out of the hospital in 15 hours and recovered in 2 weeks. Severly bruised shoulders and hips (bruises of life they called 'em), minor cuts and contusions, but ALIVE. CRUSHED the driverside cab of an extended cab Ford Ranger and laid flat the interior. The belt kept me in the cage, were the steel protected me. If I wasn't wearing it, I would have been thrown and crushed.
I'd Post the pictures, if I had them scanned.
For off-roading: From (excuse me) Jeep experience: Lap belts. The shoulder strap gets in the way. You can lean out a bit with a lap belt on, but still stay in the vehicle if it shifts suddenly. If you need to lean out farther than that, bring a good spotter... and a high-lift jack.