My partner (who is applying to vet school this year) and I have had pet rats for the last couple years or so.
I have to say that I love rats - they're quite possibly my favorite pets, over cats and dogs. They are extremely intelligent, clean, and bond to people just like dogs. However, they're much lower maintainance (just clean the cage one a week and give them food / water as needed), and it's easier to find apartments who will take caged pets. Since their typical lifespan is 1.5 - 3 years they also don't require the commitment of a dog or cat. The main thing to remember is to get at least a pair of rats (same-sex or neutered) because they're highly social animals who get lonely alone.
There are some key differences between male and female rats. Females are generally much more active / hyper, are cleaner, and are more prone to tumors. Most of the tumors can be prevented if you spay females. Male rats are bigger, much more laid back (they cuddle with you rather than run around), smellier, and more prone to aggression. The last two can be cured by neutering.
The hardest thing about rats is how soon and young they die and how traumatic their deaths can be. I just lost the last of my original pair at only 21 months from chronic respiratory issues. Her sister died a few months earlier from massive cancerous tumors. Rats are very prone to respiratory infections, tumors (especially intact females), and cancer. Because you bond so much with these animals it's hard to lose them so young. It can also be more difficult to find a vet who treats rats than cats and dogs, and surgeries can be expensive.
I suggest getting rats from a breeder or rescue rather than a pet store. Good breeder rats are less prone to illness and dying young, and buying from a pet store supports mass-breeding rats for the purposes of snake food.