I have used it a few times but with a bigger band and medium to larger room the limitations that pop up are more hardware limitations than those with Reaper. These are that for long line level instrument cord runs you really need balanced leads to the PA which is usually under the desk. Sometimes you need DI boxes on basses etc. Also mixing on stage has its own problems like needing to gaffer computers to stable little tables when there are lots of musos walking about on stage with instruments and gear. Mixing on stage for a loud gig in a medium or large room is a big no no. You will hear an entirely different mix there from what is going out Front of House.
Huh, I wouldn't like to face a live show mixing without a dedicated hardware interface. It's all real time. You must be fast and accurate. Dealing with a live mix with a mouse and a keyboard sounds like a hard way to do it. All this summed to what Gerry G said. You need a load of hardware to mimic a console. Digital consoles have it all built in, sometimes with I/O racks added. Mixing a big act on a PC sounds scary to me.
Likewise, it doesn't have to be perfect, but it has to be now... There are enough things to think about in live music, enough real and potential problems to deal with without nursing a computer through a gig. ;) All connections should be balanced to go through a multicore (snake). Not doing this is asking for extra problems. >
One of the jobs I do is multitrack live recordings for an on-demand live concert video series done by a telcom company for their digital cable service. Just last weekend I was out recording a successful international touring act. One of the band members noticed my recording rig and recognized Reaper (I record redundantly to Reaper and to an external USB drive using an RME UFX). He said that they do all of their onstage in-ear monitor mixing in Reaper using MOTU interfaces. It was my first experience encountering a working Reaper rig at a show other than my own. :D Just thought I would share...
I've been mixing live FOH on reaper for over a year now. I obviously don't mix every show on it, but I do choose to use my mix rack over a Midas Venice and the accompanying fx rack. I also have a smaller 16 channel rack that I bring into my house gig, where I have to disconnect and reconnect a Mackie console in order to use it. Like anything, you just have to put the time into sorting it out. Definitely not something you want to do if you are supplying production though. I registered fohdaw.com and when things slow down a bit, I'll get a Site up with info, videos, that sort of thing.