Didn't like it much.

For the sake of not being a complete spoilsport I'll say that I didn't really mind the length. The game's about as long as Sonic 3 was, which ain't bad.

The physics are really bad, the controls are slippery (at least on the Wii version which is what I have) and the game is just too slow and clunky. The music could have been way better too.

This video sums it up best IMO:
YouTube - Sonic The Hedgehog 4 Review

It's not as bad as Sonic the Hedgehog Genesis 15th Anniversary (the Sonic 1 port for GBA) but it's still not great.