Benefits: Puncture resistance, lower tyre pressure for better grip, some set ups can weigh less than tubed set ups
Disadvantages: Can be a PITA to get tyre on rim & inflate, if a hole in the tyre doesn't seal you have to bang a tube in anyway, some set ups can weigh more than a tubed set up
I have a pair of Schwalbe tubeless Racing Ralph on Mavic Crossland wheels, so it is a proper tubeless set up. The tyres were damn tight on the rim, but they inflated fairly easily with a track pump & a bit of sweating.
Weight for my set up is a tad heavier than my tubed set up, but that's on different wheels, so no real comparison.
I've not had the tyres lose any (significant) amount of air over about a year, they were off the bike over winter & stayed inflated to ~15psi, just added another 10psi, slung em on the bike and job's a good 'un
Never had a puncture that I've known about, over last summer & a thorny ride on Sunday (FluffyChicken got 3 in the rear and 2 in the front within 100yds)
I like em, but I did get quite lucky getting them to fit & inflate 1st time, I've not had any downsides yet, might go ghetto on another of my bikes & see how that goes.
You may find this useful: http://www.mtbtechniques.co.uk/MaintananceGhetto.html