I've always heard (from legal people who should know) that if it came right
down to, they're not worth the paper they're written on.  Their main purpose
is to make you think twice about leaving.  There is really no way they can
FORCE you to sign (what are they gonna do, fire you?).  You probably should
consult a lawyer before signing ANYTHING.

If I were you, I would, at the minimum, make sure there is a clause saying
that if they let you go, the agreement is null and void.