WELP, TIME TO GIVE MUH 2 CENTS.
Egg Mobile: Meh, similar to 2.0, and all his new attacks are easily dodged, as he can still hurt himself by ramming you while you are spinning. Either that or I just got lucky with spike placements every time.
Egg Slimer: Glass was added, and the pinch phase was made slightly harder now that the pogo stick hurts you. Not much to say, other than praise the sun that the slime now despawns at a reasonable time now.
Sea Egg: Other than the god awful zap timer and the clones, the Sea Egg is just as fun as 2.0. And while the zap timer could be extended, the clones I am fine with, it helps the boss better match the difficulty of DSZ itself.
Eggscalibur: I'll admit, I hated this boss at first since I had no clue what to do, but now, I love it. Now only does the boss put a twist on the whole, "Run and hit 'em before he attacks!" formula, but instead forces you to wait and seize the chance to rain spinning death on Robotnik's contraption (Yes, I am referring to him as Robotnik, it's a better name IMAO and is not as laughable), but the pinch phase helps make sure you understand the chain gimmick from CEZ, which, if I remember correctly, can help with finding some the other hidden emblems in CEZ.
Metal Sonic: Oh god this race annoyed me at first. For Sonic, this race is torture unless you are a platforming master, and Tails and Knux have a harder time keeping up due to their lack of a formidable top speed. However it still is doable, and leads into what is the hardest boss in the game for me. For one, he is constantly trying to fly right behind you, so often you will miss and either ram into the electric fence (which due to this makes me wish it acted like a normal fence like the electric fence in the Death Egg's Eye stage in Sonic the Fighters) or you will fly straight into the pit. Then, of you manage to reach it, the pinch phase is too damn long. He constantly dashing towards you as you are trying to dodge and just wonder how long he will keep up this attack straight from hell. So, overall, I do not mind the race, but the battle, considering it is harder than even Brak himself, it could use some dumbing down.
Brak Eggman: I like this one. The boss is still a challenge, like a final boss should be, but it does not have the annoyance and all-round cheapness of the 2.0 version of this boss. And personally, considering you can dodge most of his attacks if you try, I do not have a problem with the knock back unlike most people seem to.
Conclusion: Overall, I enjoyed the bosses in 2.1, with the exception of Metal Sonic's battle (once again, the battle I hated, not the race). And quite franky, I cannot wait to see how 2.2 will improve the bosses when it comes out, however many years that may be.