Are extension tubes worth it?

Is there a loss in quality using one of these?

I'm considering getting one and I'm wondering if it's worth it. I only carry the 24-105 with a 1:6 crop. So I hoping to extend from 168mm out to say 180 - 200. Would that be reasonable? Or can I expect too much loss in IQ.

Methinks that you are confusing extension tubes (which are used to increase the close focusing ability of a lens, in order to increase maximum magnification) and teleconverters / extenders (which are used to increase the focal length on telephoto lenses at the cost of some speed). Which were you looking for help with?

Sounds like the OP is truly interested in a teleconvertor, but the numbers don't jive...
With the zoom at 105mm, and a 1.4x teleconvertor mounted, it becomes a 147mm lens with a FOV on the 1.6 crop camera equivalent to using a 235mm on a 35mm SLR. The 24-105 is a pretty good lens, so it would have a minimal loss of quality with a 1.4x mounted (not as severe as using a 2x teleconvertor)

Could be that the OP was hoping that the EF12 would add 12 mm of focal length (168 + 12 = 180 mm), etc. In any case, there is definitely some sort of misunderstanding here.

Be careful which X-tender you get for the 24/105L. The Canon one WILL NOT work as the rear element of the lens will collide with the front one of the Canon X-tender.

I had to get a Tamron SP AF for my 24/105L.

IQ will not be lost with rings/tubes - they are full of air.

IQ will suffer with an X-tender. They have more glass, and usually are not of the same quality as your L.

Also, teleconverters will not work well with short lenses like the 24-105....they are made for longer telephoto lenses, and work best when used with them. The Canon ones will not fit the 24-105 at all.

If you are by chance looking for extension tubes (which doesn't sound like it), they don't change focal length at all, and there's no image quality loss (other than enhancing the flaws by focusing so closely) since extension tubes only contain air.

Thanks for side stepping my mistake and lending me your guidance.
I'll give the Tamaron SP a try. Hopefully I can rent one and learn something before comitting to it.

Thanks again for the kindness.