My one complaint, and it was a pretty big one, was advancing to the next patch. I just had the whole show set up as a list of patches in order and tapped through to each one sequentially. I felt like I had all the control I needed in order to make the setup as simple as humanly possible and do everything I needed. I used it for a set of shows where I was playing keyboard in the pit for a musical. I would imagine that's why Mainstage is such a bargain. It's actually causing me to transition to using Logic as I want any patches I build while writing/producing/etc to be usable both in the studio and live and Cubase doesn't like them AUs and Mainstage doesn't like them VSTs. The only downside is I find it disconnects from time to time and has to be reconnected, I have had this happen to me onstage.
I also use IK Multimedia's blueboard as a foot controller - it's super handy to have patch up/down, tap tempo, guitar mute (or whatever you want, really) there by my feet, save my hands for playing notes. Not to mention I don't have to tool around with compactflash cards to load my samples now. Tweaks to patches during live performance are easy to handle if you find you need to make adjustments on the fly. Going from one hardware synth and one hardware sampler to the Mainstage setup has been great, you have so much flexibility to load in different synths, effects, etc, with little to limit you besides your laptop's grunt. I mostly use it for live keyboard sounds with a sample pad but I've also started using it for live guitars (as an additional guitarist, whilst the right plugins sound great it's not the same as a cranked tube amp, but that might just be the guitarist in me speaking.) and the occasional effected backing vocal. I've been gigging with Mainstage for maybe 6 months now and I'm finding it works quite well.