Stop whatever you're doing and download this right now. That video above isn't just for show, this is a real demo made by a two-man team of dedicated Sonic fans. Sure, there are a few collision detection issues and some other technical hiccups, but it is still hugely impressive what two people with no budget can produce. I'm not going to say outright that it blows Sonic 4 away, but let's just say that if Sega put future episodes of Sonic 4 in their capable hands I would breathe a huge sigh of relief.
Ok, a brief reality check. This is just the Emerald Hill Acts 1 and 2 from Sonic 2 remade with (much) prettier HD graphics, along with an original level sandwiched between them. Also, there are some times when the sensory overload can get too much and the scenery will distract from things like, say, spotting enemies. But if you're a Sonic fan, then you should probably be downloading this right now.
Update: It looks like someone (I can't imagine who) had the Sonic Fan Remix demo pulled and the original Rapidshare link no longer works. An alternate download link can be found in the first post of this thread on the Sonic Retro forums. That download link will probably get taken down too by the end of the day, so act fast if you want to play the demo.