I used it in my MTB tires for a few years when I lived in the desert and it worked pretty well against goatheads. I only got one big gash that didn't seal in that time. The mess cleaned up pretty easy with soap and water and I just got a new tube. I probably could have patched it by thoroughly squishing all the slime away from the cut beforehand but figured why bother.

I'm a Mr. Tuffy guy nowadays. A bit more expensive up front but they last forever.

Oh, the clogged valve thing happened once but I was able to clear it by removing valve core and cleaning it. To avoid clogging valves always pump after wheels have been sitting with valves up in the 12 o'clock position for at least half an hour, overnight is best and is easy to do if you park your bike like that everynight then air up if necessary prior to riding.

Oh and on Ira's note about pooling. Yep your wheel balance will be pretty wonky for the first 1/4 mile or so when riding after the Slime is thoroughly pooled at the bottom.