According to it's Pokedex entry from Diamond/Pearl, it doesn't feel pain if something bites it's tail. Though I'm not sure if that applies to having half of the thing cut off.
Yummy!Aah~SlowpokeLet's have lunch here!Aah~Go, Slowpoke!Yummy!Aah~Slowpoke! Slack Off!
Slack Off restores 50% of the users HP.Slowpoke! Slack off!