I've never owned one, but I worked as a rental car agent for four years and my dad works for Toyota.
The Yaris was the workhorse of our subcompact fleet, but I would never buy one. I, personally, am extremely uncomfortable driving one and I felt sorry for the customers I had to rent one to. It feels like the car is going to tip over if you have to take a turn to hard or swerve suddenly. Even the higher end packages are extremely basic. And as someone said before, they have nearly no resale value. Plus if you get hit by anything larger, you're probably done for.
They're good for limited cars to take you from A to B, or as a rental car short term. But long term, I think you'll end up being very unhappy.
I'd go one step up and get a Corolla. They're much nicer and roomier inside, safer to drive, similarly priced, and close to the same gas mileage. The handling is like night and day compared to the Yaris, as you don't feel like the car is going to rip apart around you. It will last you much much longer, you'll get more for your money, and it'll be easier to get out of when you decide to move on. You can't go wrong with one of those.
One of the biggest complaints I got from customers about the Yaris was the gas mileage, believe it or not. If you're doing long distance, a lot of freeway, or any hills, your gas mileage is actually going to go DOWN, because the car has to work harder for it. You won't have that issue in the Corolla.
Do yourself a favor, treat yourself right, and grab a Corolla or something more on that tier. You'll be much happier. The Mazda 6 and Honda Civic are also good buys. So would a Chevy Cruze or Cobalt, but I'm not as big a fan of those as I am the Corolla when it comes to gas mileage and reliability.