Here's a great photo collage to illustrate the Sonic Adventure 1 plush. All the different views give you an idea of his quality from every angle. As you can see, he's fairly fuzzy fabric with 3D details. The body is still kind of 'oldschool' being big and round, but it has no fill problems or terrible facial features. The only bit of oddity about him is the double pointing giant fingers which are a bit strange.

These guys from this set are really rising in value now, as well as becoming harder to find as people hang onto them--so watch out if you're trying to collect. 

From the same set, there's Knuckles.
The likeness of this one may even be better than Sonic's with his smaller (non-round) body, proportionate fists, and well detailed shoes. The way his eyebrow ridge is set up gives the end of his eye a kind of 'pointy' look at the tip, but as the side-on photo shows, it's really just the view.
How big are these? You can see them with a Dreamcast as a part of someone's collection. These plushes are a nice big size with quality details and fabric.