You will lose far away focus and increase close-up magnification with extension tubes
. Image quality really doesn't change as it is literally a tube, no lens elements, just air.
You will "extend your reach"
by the multiplier (1.4 makes 100 into 140, etc). You will typically lose one stop with 1.4X and 2 stops with 2.0X, also lose some sharpness/correctness. I think this is what you are looking for.
Decent acticle on tele-extenders: http://www.bobatkins.com/photography/reviews/tcs.html
BTW - both will increase effective magnification.
(Message edited by tdeneka on March 30, 2009)