I've done it, and found out my ex was cheating. 15 mins later she was out on her arse in the rain (my flat, she was living rent free at the time). The reason I checked though was that within a week she changed her phone unlock from a swipe to a code (as if I wouldn't spot that, she could have just changed the swipe), and was suddenly going out for drinks with a girl from work I'd never heard of amongst other things that didn't add up. Anyway She left fb logged in once, I looked, sure enough she was cheating with a guy from work, so I booted her out on the street.
I find it a struggle not to check up on my current girlfriend now, but so far have resisted. (8 months and counting). She knows I find it hard not to, but she never hides her phone or logs out of fb when she leaves the room, so even though the temptation is there, she's not trying to hide anything so I don't see any reason to check.
Oh, but more importantly:
Step 1: Meatspin on full screen
Step 2: Disable the trackpad using the function key she doesn't know about.