So far I like them. On close inspection I wish they designed them to adhere to the plastic trim at the door opening, it seems like they may warp slightly and catch your shoes after a while. Might be something for me to play around with... some type of easy to remove adhesive that has just enough tack.
I was afraid they'd be too hard and thus slippery, but they're perfect. They don't prevent you from moving your feet, but you don't slide.
Oh, and one great benefit is that they don't smell. I read about some other manufacturers where people suggested leaving them outside for a week to air out. These have zero odor.