This is, of course, depending on the fact that you're looking at the same sort of tubes as I was.
- They're generally not coupled to the camera at all, that is, when you stop down the lens (which you have to do via the aperture ring), the viewfinder image darkens, since there is no coupling to the stop down lever on the tubes. This also means that you have to use manual exposure and lose metering, though I find that for high-magnification sort of work that a meter is more of a suggestion than anything else.
- They don't have the same nice baffling/flacking on the inside of the tubes that the official Nikon ones have. The ebay tubes are basically just black painted metal on the inside, and hence in many cases (especially when working with flash) cause incredibly annoying flaring. This is the #1 reason that I didn't keep them (technically I kept them, but they're in a box somewhere and never get used). A lot of times, moving the flash a little bit fixes things, but I had flaring issues with every lens I tried on them, and never had these issues again when switching to the Nikon tubes.
- They're really cheaply built. They just screw together with some pretty thin threads that I never really trusted to hold anything bigger than a short macro (like the Nikon 55/3.5, I don't know the Canon equivalents). I didn't like having my 105 on there, and it's not even the larger /2.8 one-- its the old /4 manual focus one. They also aren't machined to the same quality as a camera-brand lens, so in my case the mount to the camera felt a little looser than normal and the mount to the lens felt a little tighter than normal (to the point where it was hard to get the lens off in a few cases).
In my opinion, you are better to stay away from the ultra cheap extension tubes on eBay. Amazon has some decently priced Macro Extension Tube Sets here that I would trust much more.