It depends on what urchin and what algae you have, but, for example, in my 110g I have a diadema and green urchin and man are they pretty voracious little lawnmowers.
I had a fuzzy type algae growing over coraline on the back glass of my other tank, and a blue tuxedo urchin took care of that pretty lickedy-split. But, it's also an eater of coraline. Although I don't find that to be a problem because what happens is the spores get spread out and thus you have new coraline growth being spread. So it evens out.
I dunno, maybe YMMV but I like having 'em, I find them useful.
