Well, unless you can get GOOD packs for cheap, you're probably better off buying individual or Lots of new cells. ESPECIALLY if you are looking for Li-Ion cells cause you can buy cells with protection circuits built in.
Butchering packs is handy when say, you have an old drill, or video camera you don't use anymore, but their battery packs are still plenty good. No point letting some perfectly good batteries go to waste right ?
Many of the cheaper new drill packs you can get your hands on are going to have older tech low capacity NiCD Sub-C's in them, that . . well . . . aren't really worth the bother.
If you score a Li-Ion drill pack its most likely going to have IMR cells in it, that will deliver alot of current but again, will be lower capacity. If say, your application doesn't need that much current, you'll probably rather have some high capacity Li-Co cells.
If you score a Li-Ion drill pack that uses the larger 26650 cells, they are most likely going to be LiFepo4 cells, which, if you're looking for normal Li-Ion voltage levels you'll be disappointed. But if Lifepo4 is what you want, then it's pointless cause you can just buy them on Ebay in mass quantities for cheaper then a drill pack. (I think). There are alot of 26650 Lifepo4 cells avaliable on ebay for roughly just under $10 a piece shipped. I've bought some and can vouch for them.
If you are looking for 26650 Li-Ion IMR cells, well, those are rarer to find. At least I don't' know which company packs that heat into their drill packs.
So, point is, better off buying new cells from reputable vendors unless you have a hot lead on cheap new packs. Otherwise, everyone here would be doing it (and your question would have received some more affirmative answers.)
If you let me know you application, what kind of cells you want, and what you want to use them for, I might have some idea's to point you in the right direction to find exactly what you need.