I actually plan to look more into this myself... off the top of my head, I think somewhere between 40 and 60 is probably the best idea. One thing to watch for, I guess, is the amount of distortion you get nearer to the sides of the screen. It's pretty noticable in the stock game.
I find wider FOV's useful for just playing the game though, so it's fine out of the box when using internal cameras etc. I tried making the internal camera 60 or so degrees, and it just totally disoriented me. It felt like walking through a house with a pair of binoculars constantly at my eyes