I've very occasionally had good experiences with TAs finding me flight itineraries or prices I can't on my own, but these have been the exception to the rule. For the most part, my experience is consistent with that of milepig and Emma. I've found that at best TAs generally only do what I can do on my own or at worst screw up reservations, seat assignments and/or frequent flyer numbers.

I guess for those folks who have absolutely no time and/or absolutely no idea what they're doing, a TA can come in handy. But for those like the OP, who seems to have a sense of what she's looking for, better to do it on your own.