I picked up Sonic and the Black Knight today. Let me get my criticisms out of the way first. The controls are stiff and sluggish, the level design at least on the side missions is frustrating and in some cases broken, combat is frustrating, and some boss duels will drive you up the wall.
Now that that’s out of the way, the opening cutscene is great, the dialogue is hugely improved (though still not really good), the game looks very good, the music’s catchy, and the missions are not as repetitive as in Sonic and the Secret Rings.
All in all, I’d say it’s fair/mediocre. It’s not too bad or too good. It’s somewhere in the middle, where Sonic games nowadays tend to fall. If you liked Secret Rings, it should tickle your fancy. If not, just search for the cutscenes on YouTube and save yourself the frustration.