Some people love the Toto seats, some hate them. There is more than one model of Toto seat that will fit that have slightly different seat profiles. They are not flat, so they tend to force you to sit in the center of the thing...some like to perch on the front of it, and if you're one of those, it will not be comfortable. Now, my doctor says you should never sit on the seat for more than 3-minutes as it puts pressure in places that really shouldn't have it, so using that, I find mine are fine. There's a thread here about a few people that have had the soft closing mechanism stop working (which makes it a 'normal' closing seat). My oldest one is about 10-years old and still works fine.
Other manufacturer's seats will fit, but they may not match the contour or color exactly. The Toto seat essentially is even with the porcelain. Some others may overhang it, which isn't really a big deal to many. The only other company that makes a soft-close seat I'm aware of is Bemis. They also have a feature that lets you quickly remove it for a thorough cleaning.