Just got my first puncture last night since going tubeless at the beginnning of January. 35 to 0 PSI in less than a second! Inspection of the tyre revealed a hole big enough to poke my finger through that no amount of sealant was ever going to fix...
The tyre also showed evidence of 5 other small sealant plugs which may have meant flats if running a tube set up (and certainly would have been flats in a tubeless setup without sealant).
Morals of the story:
-sealant works, most of the time
-for those times it doesn't work, you really need to be carrying a spare tube and a tyre boot
-even on a trail you know well, don't follow your buddy too closely especially in the dark
-a finger-thickness green branch with a pointy end can punch a finger-size hole right through the centre of your tread
On the plus side this means I can get rid of the Monorail tyre that I didn't like much without feeling too guilty