Originally released in 1996 as a two-part OVAs (Original Video Animation), Journey to Eggman Land and Sonic Vs. Metal Sonic, the movie was dubbed and released into the English world in 1999 as an hour-long movie, featuring a whole new world and new friends and a new threat that's been with the series since 1993. Although not a bad movie, certainly more entertaining on the whole than Sonic X, the problem I find with it is that it came at a bad time.
The Sonic series had kind of died after Sonic & Knuckles, and went into spin-off limbo where the main story was withheld while the likes of Sonic R and Sonic The Fighters sprang up, which were pretty decent games, but not exactly the winners of five flaming heads in game magazines. And then he was essentially revived with Sonic Adventure, given a new attitude, new looks and some new buddies to hang out with; new everything, essentially. Just about everything from old Sonic was revamped or changed in some way.
And it was only then that they decided to get the movie over.
Sonic the Movie keeps up tradition by bringing Sonic into a new world with new allies and new locations, just when those exact concepts were being brought into the real series, only this time muddling it up with more fantasy ideas than the real world, and making the president into an overweight elf person thing with a shrill voiced cosplaying daughter, it all gets a little bit crazy and complicated. Considering Adventures of Sonic the Hedgehog featured Sonic and Tails hanging out with a Professor Caninestein and Robotnik having a mother who looked exactly like him, it's not that far-fetched, but I'm sure some people were confused. At least, I hope some people were confused, otherwise my argument about wrong-time-wrong-place would be bunk, and that's precisely why I bothered with this section!
Since this argument isn't quite going anywhere, I would normally go on to say that perhaps it was a failure (I think!) because of lack of interest and advertising, with Sonic's hibernation and all, and the fact it was released direct-to-video in the United States. However, over here in the UK, it was actually a cinema release! I think there may actually have been a television advertisement for it! And naturally, with a whopping PG rating, we imagined it would feature all kinds of EDGY things like Knuckles DYING or Sonic SWEARING, but we ended up not seeing it until I got the DVD, where it was more of a "haha we were excited over this" sort of thing. I'm not sure where this is going either.
In a nutshell, the movie is pretty decent if you accept that it's apparently impossible for cartoons to be about Sonic without throwing him into a whole new environment, and if you can tolerate that it isn't Adventures of Sonic the Hedgehog. Nothing can top that show!