extremely high, the game's pretty much built around replay. i've manage to unlock 3 ships out of the 30, it's quite addicting since you basically go 30-60 minutes games, and every game is interesting. You think your doing good, then you jump to a star, and just get wrecked, or deceide to help some people with a giant spider problem, only to have one of your crew killed off(or sometimes you succeed).
the possibility's of how to approach combat is also really great, the weapon choices, ship layout's...it's just such an addicting game imo, I highly recommend it.
edit: also, i still haven't managed to even get to the boss, I got to the last sector once, but the first guy i fought destroyed me.