Personally, I have one and don't use it. When I play my C64 I don't mind if it needs to load for a bit, and disks don't take *that* long. It could be that my drives are already enhanced in some way I'm not aware of, though.
If it's cheap, though, I'd say go for it. There are alternatives, though - JiffyDOS on the disk drive, alternative third party disk drives, and other fastloading carts like "The Final Cartridge" (I have version III). It's not really as simple as yes or no, heh.