Sonic Adventure was always pretty bad. At least I remember returning it a week after I bought my Dreamcast. Not even the Sonic sections were particularly fun and it came out right in the middle of all these great other great mascot platformers. You could pick out something random like Rocket N64 and it was a better platformer. I liked Chameleon Twist and Bomberman Heroes more than Sonic Adventure.
Though, Sonic Adventure 2 and Unleashed are fun. I heard Colors was good but when I played it, it was not.