Yeah, JiffyDOS is well worth the money. Even a C128 with a 1571 will get a good speed boost. The DOS wedge that you get with it is also very handy. And if some of your older software don't like JiffyDOS, it's easy to disable. (Flip the toggle switch and reset.)
I've used it quite a bit and I consider it one of the almost mandatory upgrades for the old CBM 8-bitters.
