A.V.
We've used one 2x for big trips to the UK. We paid the accommodations through them and they got kickbacks by using particular hotels, etc. I will say that I will be unlikely to ever again use one for a complicated vacation (multiple locations, car rentals, flights, etc.) The first time, we got to the place we were supposed to pick up a car well after they closed and could not get to our hotel 40 miles away. We had to pay for another room in town as well as the one we didn't sleep in.
This time, we ended up with a red eye flight for our family leaving Dublin last summer, which stank, because the travel agent messed up. There was so much back and forth that we might as well have done it ourselves. So I have not been impressed. This last time the other thing she did was make us return cars to airports, vs other locations, so we wasted a lot of money trying to then get our family to the right place for the next thing (like the ferry). At one point we were almost left at a bus depot miles from our hotel. She did find us some nice apartments to rent for a few nights, so that was good, but I am overall meh on travel agents. We discovered later that she was outsourcing as well, so it was like a giant game of telephone. We only used one because DH insisted. I feel that if you do use one, use it for not very complicated vacations and follow up on every detail (at which point you might as well do it yourself).