Rats aren't as bad as everyone makes them out to be. As long as you don't get one of the super nasty feeder breeders from the petstore that've NEVER been handled, you'll love them. They have their own personalities, they aren't stinky if taken care of, and the marking that they do, it's just a little streak across your arm. Think like dipping a q-tip in water, tapping it somewhere to remove excess water, then dragging that a few inches on your arm. Really, it's not like they pee gallons of pee on you. They can also be potty trained (but, do expect to be marked with pee, there's no getting around that). If they're properly socialized (again, basically, as long as you don't get an abused rescue or old abused feeder breeder) you shouldn't ever get bitten. They're not like hamsters that will bite simply because you woke them up. They're also extremely intelligent. My very first rat, Kilo, was trained like a dog. She would fetch things (ok, she didn't do it on command, but if you threw money, she would fetch that and bring it back), she was litter trained, she played hide and seek, she came when called, etc, etc. She was a thief though, you had to lock your money up, or she'd steal it and stash it away, dollar bills and coins. I found a ton of "lost" money by cleaning out my book shelf, lol. If you were paying attention, she'd bring the money to you, but if you weren't, in the bookshelf it went.
Dumbos are said to be, in general, more laid-back than standard rats. Other than this, and basic appearance, they really aren't much different from other rats. You can get your nasty dumbos just like you can get nasty standards.