I've been doing a lot of research on them lately to build my X-Men 4P into a Konami 4P machine. I already have X-Men and TMNT, still looking for Simpsons, Sunset Riders and Captain America and the Avengers (not Konami, but harness works the same). I still need the Jamma 6-in-1 switcher to so I've been keeping an eye out for deals in regards to it.
The one on eBay will cost you $95 when all is said and done. Jammaboards.com sells theirs for $99.95 + shipping but you can have them install the -5v to whichever slots you need for an extra cost. That way, if you know a certain PCB needs the -5v you can have it ready to go when it shows up.
To my knowledge the boards I'm looking to play don't require that voltage, but I haven't found concrete evidence yet (can anybody confirm/deny that for me?).
Also, if you're using games with kick harness or Jamma+ support then you'll need to use diodes on the harnesses so the feedback doesn't effect your games. There is a good tut on this board if you search for it. Hope that helps.