Positives:
Get healthy
Rest
Get to go home for a few days
Few extra days to work on fundamentals
Few extra days to work on new plays
If you have lost a few games prior to the bye it gives you a chance to reboot

Negatives:
You get rusty
You come out flat and if the other team punches you in the mouth it hard to get going
If you have the momentum with a few wins going into the bye you can lose it