I get the 'grossness' opinion Doglover. At least from the point of view of someone who's never owned a rat, or who's only experience with rats are seeing feeder rats kept in unhealthy and dirty conditions.
I mean seriously, lets compare:
Apples to apples to the uninitiated, rats are visually the least appealing of the rodents: Big muscular bodies. Small thick ears in relation to their body size. Large thick scaly tails. Slanted eyes that bug cross eyed when they look at you. Big clawed oversized bald feet. In fact, they look closer to creepy reptilian possums than adorable housepets. Not to mention their reputation for causing the bubonic plague which killed off up to 60 % of the world's population. A fact that still holds such a cultural stigma and fear in our minds that we can't even look at a rat without thinking we've caught something noxious.
Rats don't have the big adorable eyes and lion fuzzy tail of gerbils. Hamsters have the benefit of not even having a potentially off putting tail and also having little round chubby Buddha cuteness, unlike the ratty large muscular strength. Guinea Pigs, while big and massive like rats, also have the plump fuzzy cuteness and don't have the thick snakey tail.
Rabbits, while big and muscular, have a powderpuff tail, big round eyes, and long adorable ears. Rat ears are among the smallest and stumpiest of rodents.
Even mice, who structurally are the closest to rats in form, benefit from cute button round eyes instead of eyes sly and slanted. They also benefit from dainty proportions, which make their snake tail look less scaly (though it is every bit as much if you look closely). And they have huge delicate ears that perch cutely on their fuzzy heads.
Many people are put off by rats even if they like rodents. But, for me, once I researched them and kept them, for my wants and needs, I have found them to be much more enjoyable as a pet than any other rodent I've ever kept. Everything Vanillarat said, I've found to be true.
If they're not made to wallow in their own filth like they do at pet stores, rats are cleaner than and absolutely less smelly than all of the other rodents. Mice reek the worst of all of those. Rabbits are stronger smelling and not as clean, but stringent cleaning can keep the odor down. Unfortunately, nothing helps mice. LOL
Norway rats (from whom pet rats descend) are incapable of spreading the black plague--black roof rats are the once susceptible to it. Domesticated rats do not spread any kind of illness to people any more than any other domesticated rodent from a pet store.
They are hands down problem solvingly smarter than every other rodent, though rabbits come close.
They bond, enjoy, and play with their humans. Rabbits do this to a slightly lesser degree, but I've not seen any other rodent do this.
They are less destructive than rabbits and gerbils (gerbils are the most destructive and therefore most likely to be escape artists). Guenea pIgs are not very distructive at all though.
If they do escape, rats easier to catch than mice, gerbils, or hamsters. (Gerbils being the hardest to catch). You can train rats to come when you call their name, like you can with rabbits.
Aggressiveness: While all animals bite, rats are the least likely to bite of all of the rodents. I've been bitten by several of my mice, all of my hamsters and gerbils, and bitten and aggressively scratch attacked by my rabbit. I've never been bitten or aggressively scratched by my rats or by guinea pigs.
Noise: Rats are silent unless something's wrong. Guinea pigs are noisy. I've not had noise problems with any other rodent.
I would compare rats cost and otherwise most closely to guinea pigs and rabbits. Rabbits and guinea pigs have the edge when it comes to cuteness and longevity. Rats have advantage on rabbits when it comes to odor, destructiveness, and aggression (rabbits stink worse, destroy more, and bite/scratch more). Rats have the advantage over guinea pigs when it comes to intelligence, human bonding, and entertainment factor.
As for looks, I wasn't attracted to rats for their looks at first. However, they've grown on me, and I now find them adorable.
In the end, as Vanilla said, it all comes down to what you prefer in a pet, which is an individual choice. You may research rats yourself and still decide that their looks are off putting enough that it doesn't make up for their intelligence, cleanliness, and bonding abilities. Or you may decide that for a shorter lived pet, their relative cost isn't worth the expenditures compared to a rabbit or guinea pig. You may also decide that their social need time requirements, isn't something you want to fool with and you actually prefer a rodent that doesn’t need as much from you as an owner.
Or you may be sucked in like me in spite of yourself.
I love all the rodents I've had, but rats happen to be my fav.