i've had a few good pulls lately using M/S slightly FOB at mid size outdoor shows. i think they came out great! using both hypers and cards for the M.
Inside venues?
I've run it inside and outside. Mixed both on the fly and in post. Here are the few pointers that I'd give to someone whos never played with it. Do it in post to start with. That'll give you a good idea when you run on the fly about the mix you like. I would start with 50/50 (that is the same gain on both the mid and the side) and move from there. I usually go with a tad more mid then side, but it depends on the place. Outside usually calls for a little more mid, especailly because you are a little further back. I wouldn't really try more side then mid outside, but to each his own. I was extremely close at moe. acoustic for bonnaroo and was running them about 3 db from each other with the mid a little higher then the side.
For inside, I would also suggest starting at 50/50, but then play with it to your liking. Bring some headphones. Depending on how close you are, a touch more mid might be nice.
Remember, if you don't want to mix on the fly, it keeps the sound very open to whatever you want to do. In post, you can make your recording sound like cards, hypers, or mono, and have as much depth as you could possibly need. I really really like M/S and have had nothing but good results with it. Its incredible what you can do with it in post if you record the mid and side to separate chanels, makes it so versitile and really opens the soundstage.