The major problem with any overlay like Sense is the degree it changes the stock android experience.
For example Sense completely changes the behavior of the 4 buttons on the bottom and the menu's as well. So if I go to my fathers who has a HTC and he asks me to fix something - I have no clue how to navigate the menu's. It's annoying and it makes me want to hurl it across the room.
Moto's Blur adds a few nice apps and some bloat. That's it. Otherwise it's stock.