Instruct: Classify the given review into positive or negative sentiment.
Query: it works but....

The unit get nice and warm, look to be very safe for the animal. but, the bunnies don't use it. the plastic materiel is too slippery and the bunnies don't like to go on it. 30*F in the last couple of nights, the units are on and the bunnies just ignore it or jump over it.