I used it on stage at the Pistoia Blues Festival last year. No problems at all.
Had to fly to the gig of course and my planned stage weapon, my then new Roland RD700GX in its flight case was 40Kg. Not allowed to fly even though I had paid for an instrument to go. I discovered this just hours before setting off for the airport.
I was OK for piano sounds as I had an actual piano (yamaha P70) already over in Italy, but organ sounds were an issue. I have a stack of synth modules but the only way I could get the organ sounds that I wanted was iMac with Mainstage and a CME keyboard - and the total weight was well within limits. The CME uses a WIDI connection so in fact my iMac was off to the side of the stage and I controlled sound selections from the CME. No latency at all.
http://www.thecompassroses.com/gallery/photo_list.asp?l=1&c=5&g=Pistoia Blues 2008
I since invested in a Nord Electro to be able to carry something much lighter and get the Hammond sounds! Of course MainStage gives you a much wider - almost limitless sound palette, but make sure you get your pre-sets nicely organised. You don't want to be fiddling about with a mouse during performance.
