One reason you don't see a lot of older Hyundai Sonatas on the road is that they just weren't that popular until maybe 3-4 years ago. With the latest 2006 model, they seem to be suddenly all over the place, and everybody's darling.
Reliability on them isn't so bad, but they do have a lot of things going against them. First, there's perception. People remember those nasty Excels from the 80's, and other models they foisted on us in the 90's that were junky. It's very hard to shake a bad perception.
Secondly, because Hyundais are so cheap, they appeal to a large demographic that's credit-challenged. They often get bought by people who have no business buying a new car in the first place, so they get repossessed, or traded on something else with the negative equity rolled, etc. And they also appeal to a lot of cheap people who just abuse them and run them into the ground, without taking care of them.
The only thing that would really scare me away on the Hyundais these days is resale value is something came up that I had to sell the car prematurely. But then, at the same time, if I were to buy one it would probably be at a deep discount anyway, so there's some consolation.
I've sat in the new Sonata, and do have mixed feelings about it. It seems pretty roomy at first, but in the back seat the wheel wells really cut into the passenger cabin, turning it into essentially a roomy 4-seater. Try to get three people across, and it may have the shoulder room for it, but you're going to be bumping asses!
The interior also seems much more plush than something like an Accord or Civic, with more soft, padded materials. But at the same time, the plastic parts seem a bit cheaper. It's like they took an interior that may have originally been a cut below an Accord/Camry, but then tried to spruce it up with some thicker, ritzier materials here and there. That's a trick that American Motors used to do in the 70's with its cars.