I actually played this after its sequel, but despite being simpler I was not disappointed.
Like its sequel, its core strength was in having a variety of game modes: one per character, six in total.
It also showed the same story from different viewpoints, in this case six different viewpoints (one per character) as opposed to its sequel's more streamlined two (hero and dark). Though it likewise had its 'true' ending unlocked by completing all the viewpoints.
One of my favourite things about this game is that you get to play as some characters other than the obvious 'posterboy' trio of Sonic, Knuckles, and Tails, which in some cases are more interesting and more fun, both storywise and gameplaywise.
E-102 Gamma remains one of my favourite Sonic characters, and his story remains one of my favourite storylines of any game, being a bittersweet exploration of themes like innocence, morality, selflessness, and self-sacrifice.
I like the simplicity of Big's storyline, which consists entirely of an overgrown, slow-witted purple cat bumbling around in an attempt to rescue his best friend. I'm probably one of the few people who didn't dislike Big's fishing mode, but it helps that there's only a handful of levels, so it doesn't outstay its welcome.
Amy's mode is also quite interesting. Firstly because it consists of trying to evade capture (and storywise actually failing to evade capture at one point), and secondly because Amy's hammer is an unusual thing to be able to use in a Sonic game. Her interactions with E-102 Gamma are definite highlights.
Another thing that makes this game interesting is that it has an overworld that gradually opens up, holds various character powerups, and later has various challenges avaiable. As far as I'm aware, this was the first (3D) Sonic game to do that rather than merely having level-navigating menus or (worse) being just a constant stream of levels.