I'm with "whittierider", as I always ride on aerobars. His tips are accurate.
I'd been riding a TT bike, as my everyday ride. I wanted something that could traverse crappy roads, so I started looking at cyclocross and touring bikes. I knew the layout dimensions of the TT bike and took those with me, when I went looking. I ended up choosing a frame one size smaller than normal for a road bike, for my height. This allowed me to maintain the horizontal dimension that I knew worked. The great thing about the 'cross bikes is that their headtubes are taller and that allows the bar height to be higher, relative to the saddle. In a addition to the extra headtube length, I added a 125 degree stem, to the mix. I've ridden with a rear rack and overnight pack and didn't notice any difference in the aero position. So, now I have a comfortable, mileage bike that can ride anywhere I want. Oh yeah, I have fenders to keep the crap off me, too.
As for what brand/model of aerobar to go with, I like Profile Design T1+. Every single aspect of them is adjustable fore/aft, up/down, and rotationally. Plus, they mount the extensions above the base bar and the elbow pads mount above the extensions, which allows a higher stack height than one can with some of the other offerings. Profile offers replacement parts, too. You want the most adjustable, highest stack height aerobar you can find for comfort potential. It'll probably take a few miles of fiddling with them to get them where they're perfect.
Many of the Great Divide mountain bike racers use aerobars. The movie,
Ride the Divide, shows several people utilizing them, but, doesn't mention anything about setups to allow that on a MTB. The toptube dimension is roomier on them and makes setup a little more difficult. For sure, I can't use them on my current MTB.
This is my setup, still utilizing the stock stem.
That's an Aerodrink bottle holder bracket in the middle.
Hope that helps.