Rats are by far one of the most intelligent and social animals out there. It's almost a shame to use them for feeders, but we do (I said "almost"!) ;^)
I had a rat here a few months ago that I had meant to euthanize with a bunch of others (was just waiting for him to get a little bigger) & then I forgot to euthanize him & he got TOO big. I brought him in to show hubby, who exclaimed, "WOW, what a monster!" Two-year old (human child) went crazy over him (he's a pretty beige), and we decided to let her "keep" him for a pet. Now, mynd you, that this guy had been pretty much on his own for a couple of months, not messed with, not played with...just fed & watered...but he is the most MELLOW & sweet rat you could ever want. (In fact, you can do a search on youtube for "Emily & Her Pet Rat" & see!) I don't know if it's just good genetics or the fact that he was lonely for so long, but he just loves attention. Whenever I stick my hand in to feed or mess with stuff in there, he just sits and waits for me to pet him (he especially loves the "ear rub"!)
We try our best to breed for personality as well as looks (even if they are mostly feeders, I still want nice & pretty rats!), so if I have one that is particularly anti-social or bitey, I euthanize them rather than letting them continue to pass on that trait. Some of them, of course, just don't care for the initial "pick up" and that's okay, they're just not used to the handling, but once they get used to that, they should be fine.
If you want to see a great demonstration on the intelligence & trainability of rats, while you're at youtube, you can do a search for "trained rat", "rat training" or "rat basketball" (Yes, they can even learn that!)