Let me first say that I never played the original Castle of Illusion, so I'm judging this game by what it is on it's own.
And what it is is a rather short but all right platformer with excellent presentation. The storytelling aspect, narrator and all, is pretty good. The level and enemy visuals are appropriately creative and "magical", evoking that early "Harry Potter" feeling of wonder. But the gameplay itself is "merely ok".
Castle of Illusion is pretty easy for the most part, though the difficulty does ramp up toward the end. I died a few times to almost every boss while figuring out the patterns (I'm a bit slow when it comes to this), and had to use a continue on the final boss.
I had some strange drops in performance in most levels - strange because while my system doesn't quality as a "modern gaming rig", an i7-920 w/HD4850 shouldn't have had any trouble running a game with these graphics - which while rather nice looking, aren't exactly complex. And this was with Anti-Aliasing disabled.
It should also be noted that this really is a very short game - it took me just under 3 hours to beat. It would obviously take longer to 100% it (get all the gems and other collectibles in each level) but if you're like me, you don't care about that kind of stuff.
As such, I'd recommend waiting for at least a 75% off sale. I think I got my $3.74 playfire reward's worth out of CoL, but I'm glad I didn't pay any more than that.