I have not ridden tubeless, but I have several friends that have.
On the road:
I've heard nothing but good things. I have a buddy who is an editor at VeloNews who got a set for product testing and he can't say enough good things about them. Whenever he meets a fellow roadie, the first thing he asks is "Do you ride tubeless?". He raves about how low a tire pressure he can run and, because of this, how much traction he gets during hard cornering.
Pretty much the same thing. I know a few local racers who live and die by their tubeless wheels/tires. The only real disadvantage seems to be the weight, which isn't that much all things considered.
Road tubeless seems fine, but you absolutely need tubeless-specific tires. And I'd still carry a spare tube plus something to boot the tire if the sidewall tears.
I use tubeless for mtb, but only on a UST or Stan's rim and with a UST or "tubeless-ready" tire. I carry a spare tube and tire boot.
I personally wouldn't use tubeless for cross. I've heard and read too many stories of people "loving" their tubeless set-up only to have it fail at the worst possible time. And by "fail" I don't mean they happen to get a flat, I mean the tire just up and comes off the rim for no good reason while JRA.
You can still use sealant in your clincher or tubular setup. Sealant won't seal major pinch flats, but will seal small punctures (i.e. goatheads, etc) and it's a good insurance IMO.