An extension tube has no glass in it so the image doesn't degrade. The lens retains the same level of sharpness as with no tube fitted.
The only thing you lose is the amount of light entering the lens - usually around 1-stop but it does depend on the thicknes sof the extension tube. You'll still have full autofocus/expsoure - provided they're atuo tubes.
The benefit gained is a shorter minimum focus distance and a larger image size on the sensor.
I'd say give it a go.
Mick, this depends what you want to shoot. Do you want to do proper macros (small insects, close-ups of a butterfly's or dragonfly's head) or just get closer to your subject? I don't know the very much about your lens, but would you say it was a very sharp lens?
Almost all true macros have very sharp optics, as macro pictures need to be sharp and crisp to look their best IMO.
If your lens is sharp to begin with then it will likely produce good close-ups, but no doubt a dedicated macro lens will be better if you want to spend the money. I bought a tube for my 100-400 zoom, but still bought a macro lens and it is definitely better.
Steve
edit: looks like Ian beat me to it, but we both said more or less the same thing
Thanks both. I have the 100-400 and use it for my Dragon fly images, which are quite acceptable, but not so good with smaller Damsels. As I say I have the 70-300 doing nothing and was wondering if it would do closer, like Dragon fly heads etc. It's either that or trade it for a macro lens. If i did could you recommend one?
Before you go and spend oodles of cash on macro lens give the Raynox DCR250 or DCR150 a try. Check the entomology page on my website for othe images.
I find exactly the same with the 100-400 and Damsels. I think either the Sigma 150 or 180 would serve you well (I have the 180), but before you splash out take look at Walwyn's suggestion...(not sure about the working distance though?) Some dragons won't allow such a close approach as you seem to need with the Raynox adapters.
As I say I have the 70-300 doing nothing
I could make you an offer for it.
James