Lower pressures, less ride stopping punctures. Downsides takes technique getting them on, and if the puncture doesn't self seal its a pain in the backside getting a tube in. I thought I would never be able to get one in at the roadside but I did it the only and only time I had to in the rain and dark took me 25 minutes though! Some people seem to go a long time without punctures I seem to get a few on clinchers but only had one ride stopping one on tubleless in around 5k, all the others I jump off and add some air and then ride again.
My preference would be a tubeless ready wheel. Schwalbe believe they are the future. Tyres are expensive.
I converted after coming off quite badly when my clincher blew out. I genuinely think I would have held it had I been using a tubeless tyre purely because I am quite certain that it wouldn't have blown like it did and also it would stay on the wheel and stop that rim metal on tarmac lack of control, that's enough of a reason for me. Oh and you get a lovely schwishing sound out the of the saddle.
http://www.schwalbe.com/gb/tubeless.html