Yes - so long as you put them on a decent lens I'd say so. I have both the 1.4x & the 2x TC for use with my 70-200mm. have used both very regularly and been seriously impressed. Makes the whole shebang mighty heavy though! A bit over an hour of Rugby with the 2x on and my shoulders are screaming at me!
Not all lenses will accept them though....I think most lens manufacturers have a list on their websites of what can and can't be used with one. Also worth remembering (assuming you're canon which I believe you are?) is the fact that anything other than 1 series bodies will lose AF with a TC over f5.6. having said that, while we were away I stacked both TC's and not only did I keep the AF, the camera cheerfully insisted that it was wide open at f5.6 - not sure how I achieved that!