we carry a Safety Seal Tire Repair kit. Check-out
http://www.expeditionexchange.com/safetyseal/ for a write-up.
I had the same thought before a trip to CO on whether to take the spare with us or not. Just for peace of mind, we took the spare with us as we knew we were going to encounter rocks.
If you rip up a sidewall, the kit can not repair it. If you just puncture it, it works great!
hope this helps.