I do both. I have purchased some and tweaked them to what I needed, and I have created some from scratch...you can usually tell the difference in my animations (soon to be made available via youtube!)

I usually only by packs that have motions I am having trouble doing myself (as I'm still new to this) like backflips, or sword fighting (still waiting for a brilliant jedi vs jedi one to buy lol) but I recently purchased the M3 talking pack that went on sale recently and it has come in very handy. I always end up tweaking the animations with it to fit the scene and flow with my other animations a bit better, but it shortens the time process in doing those scenes enough to have been worth the money.

I've also done a bit of BVH importing to do my animations, but those usually take up more time than less in getting the animation just right. I only do that when I'm desperate for a movement I just can't seem to get on my own, and it is used more as a guide in doing it.