My just turned 13 yr old is midway through year #2 of allergy shots. He has had a LOT of improvement. Before starting them, he would go through a box of tissues every 2 or 3 days. Sometimes faster. And this was year round except for about 2 weeks at Christmas time. He has so many medium reacting allergies (a lot of pollens to grasses and trees) that bloom at different times of the year that it just goes on all year long....
Now, he is down to a box of tissues a week or week and a half. He no longer has the allergy "crinkle" on his nose. And he can sneak up on someone without them hearing the tell tale "sniff" before he surprises them. :) Seriously... he can get through a sporting event without needing a single tissue (most of the time). And if he gets a cold, I can actually TELL that he is getting sick and not wonder if he is sick or it's allergies. Actually, he hasn't really had a cold since he's been getting the injections. We opted to give them a shot (pardon the pun) after he came down with pneumonia from one of the "gee, I wonder if it is a cold or is it allergies" episodes. Turned out it was a cold...well, actually, double pneumonia. :(
It's been a HUGE quality of life improvement for all of us. :)
ETA: If you are concerned about the "shot/owie" factor, don't be. Our son gets 2 injections (one in each arm) each time (he has a wide range of environmental allergens and the stronger ones are in one serum, and the other milder reacting ones are in a 2nd serum = 2 injections). They use the teeniest tiniest needles and he never complains about them. He doesn't care much for the itch at the injection site for the first 5 minutes or so afterwards, but other than that, he is good. He actually sort of looks forward to going...he gets to sit in the cool gaming chairs and play video games while he waits during the monitoring (you should expect to stay in the dr.'s office for about 20-30 minutes after each injection so they can monitor any reaction).