Both...I'm not normally ambidextrous, but I switch from one to the other anyway. Just because. It probably helps ward off repetitive stress injuries too. Although I think I've been using my left hand for a week straight now.
there are things i do with my left (like eat), but i mouse with my right. even if i wanted to mouse with my left, i couldn't; my mice are contoured for righties.
Depend on which machine I'm using at that instant.
Normally right hand, but ..
If I'm sitting with my laptop and reach over to grab the mouse on my iMac, I'll use my left hand.
OR if I'm eating I'll use my right hand to eat and my left to mouse.
Having three machines on one desk kinda breeds this behavior. I can similarly type fairly competently with either hand alone.
I'm right-handed, but mouse w/ my left hand (since Fall of '06) b/c the repetitive stress on my right hand was starting to cause me pain... Sucks, b/c it makes a "nice" mouse like the MX Revolution pretty much unusable for me...
I'm a lefty when I write or draw, but I use my right hand when I use a mouse. I wonder subconsciously how that affects my thinking b/c if I use my right hand to design on stuff on the computer, am I using the left side of my brain? Isn't the ride side of the brain more creative? Anyhow, I've had repetitive injuries using a computer too much so I've had to try to use my left hand for the mouse and even try to use a wacom tablet.