I loved their music in Sonic Adventure 1 and 2. Live and Learn, It Doesn't Matter, and Open Your Heart are fantastic. Jun Senoue's soundtrack for that game was just as great. Sonic Heroes is also good.
All of this stuff was good, classic hard rock-heavy metal, and it actually got me interested in metal, which helped define my taste in music over the past decade. It's worth adding that Open Your Heart and Live and Learn in particular still earn a spot on my metal playlists, right between Dream Theater and Iron Maiden on one of them. It fits, and the songs are that good.
After that, though, I actively disliked the Shadow the Hedgehog theme. It's like they were trying to sound like Godsmack, Disturbed, or some other WWE rock band. It didn't sound exactly the same, but it had a similar vibe. Maybe it appealed to the demographic (after all, the whole game seemed geared towards 13-year-olds who thought saying the word "damn" was cool), but to me, it just sounded childish. Now, I think it's tolerable, but still a far cry from their good songs. It's probably not their fault.
Haven't listened to their new material since then.