This is the game that inspired Undertale. The entire premise of the game, the character designs, basically everything is the cup from which Undertale drank from. It's not quite as refined as it's successor -- it doesn't quite hit that level of deep lore and charecterization, but it is a brilliant game and truly a trailblazing title especially for an indie title in 2004 when it was released. I love this game!
I've never played sonic nor particularly undestood it's appeal. I guess now I do. I really enjoyed this game, it's got tight mechanics and it's extremely well polished. Furthermore, its got great music and graphics which, combined with the gameplay, makes up for a fairly generic story. There are some glitches but they are just annoying and not game-breaking.