One hand pretty much exclusively. I almost always use my right; I might occasionally use my left if I'm needing to hold a key with my right hand, but I that doesn't come up often. It really just fit on the desk better to put it in the middle. At first I thought it might be more ergonomic that way, but I don't think it really makes a difference.

(This is OP, by the way, for some reason your comment didn't get forwarded to my Mastodon account that I originally posted from.)