I think some of them, such as Piplup and Shinx, were awesome, but some of them were stupid. I think what I really dislike about the Sinnoh was the sheer amount of evolutions they added. From what I could tell, a large percentage of the Sinnoh Pokemon were just evolutions of previous ones. Some of them were cool (I love Gallade, Mismagius and Leafeon/Glaceon), but some of them were really unnecessary. Lickilicky is a great example of this.
Also, too many legendaries! Hoenn was already pusing it (I never understood the purpose of the Legendary Golems), but Sinnoh pushed it WAY over its limit. Thirteen? Really? I wonder how many legendaries Gen V is going to have...