I bought the Tascam DR 50 because I like to record live shows. What I like the most is it has the XLR inputs as well as the built in mikes and you can record 4 tracks. You sort of need at least that for live shows.
As said the output of a FOH mixer will be vocal hot. And it's a crap shoot to "guess" at a sub mix without being isolated from the room. So ultimatly you capture as many seperate tracks as possible if you need a great mix. Some sound people don't want you to have the board mix because you go home with proof they might not have done a very good job.
I get almost all I need if I set the recorder up at the FOH mix possition up hi on a mike stand, and then steal a board mix. I'm doing the sound so I cheat a little and try and push the backline and drums in the mix.
It's nice if there is spare AUX channels and then I can say, put the vocals in the left and the Bass and Kick to the right. The ambient mikes will be stereo and the little recorder sould pick up what you are hearing. Having the direct board mixed in with the ambient tightens up the sound and buy nudging the tracks time a hair you can get good results. Depends on how far back you where.
For the best results a multi track recording cannot be beat. But to do this you need a mixer with direct outs on all channels and an audio interface that has enough inputs.
One other option I've used was to ignore the PA system and add your own mikes to the stage.
You can do a lot with 8 good mikes.