I'd say buy Jamstix - I've been a very happy user since V1, and while there was some resistance, on my part, to V3, I'm pretty happy with it now. I still think the V2 interface was a tad easier to navigate, but that's picking nits.
As has been mentioned, there are two ways to use Jamstix:
1) You can let it write the part. It is still a rules-based approach, but it certainly seems to 'listen' to the tracks you ask it to listen to. Even if you don't use the 'listen' feature there is a TON of flexibility in the rules.
2) You can use it to humanize an existing part. I would put it at the top of the list of humanizing tools. And again, you can control how much it changes with the rules.
Mostly I work with patterns that I've roughed out ahead of time. But I have just let it rip, and I've been very happy with the results. Not to say that there was no tweaking, but it was still quicker than starting from scratch!