As someone who uses sampled drums, I think Zombass for sampled bass just isn't the same as real bass. It is great though if you don't want to constantly shell out cash for new bass strings when recording, because once your bass strings get dull so does your tone. I find it doesn't mesh as well with the guitars because you don't have that high end anymore.
Anyways, that's a long way of saying Zombass is pretty good, but you better spend as much time on finding your bass tone as you would mixing your drum samples or else it's just gonna sound robotic. Though sometimes that's what I want in a mix!
If you have the money, then Trillian is the top of the game. If you're into metal(Sturgis)core, then perhaps Texas Grind Bass is for you. Zombass 3 works wonders for very low-tuned stuff, Zombass 2 actually has some instruments free, if you have Kontakt!