Rats are absolutly NOT nippy, unless they have never been socialized. The one rat I have ever been bitten by was my boy Torgo, who had a very traumatic past and is having a lot of trouble accepting people. Every single other rat I have owned/handled has been completly non-vicious, even when they were scared.
But also, rats are not cheap to keep - Only to buy. Considering they really need to be kept in pairs/groups, for a cage of adaquit size you'll be spending at least $50 bucks for a new one, or 20 or so for a used one. Also, unless you get then from a recognised breeder, females have a high chance of tumors, which need to be surgically corrected. In my 3 years of keeping rats, I've spent about $3000 on medical bills ALONE.
Getting a pair of males can run cheap - My 2 current boys are in a hand-me-down cage and in perfect health, so I have spent very little money on them at the time. But like my past rats, they have the potential to get very expensive in a very short amount of time.
*cough* Sorry for the little rant. But even though they are cheap, they are not a cheap pet. I would never, ever reccomend them as a pet to someone who does not have a job and money to spend.
However, if you want a pet that doesnt' smell, and is very easy to care for... I found Spiny Mice to be quite nice. However, they are jumpy... But very friendly.