I found my 24-105mm L to be much better than the 3 or 4 28-135mm's I've owned. There is a law of diminishing returns, you spend a lot more money for a relatively small increase in IQ.
However, the build is worlds better, AF is fast, and it likes you to use at at f/4 rather than having to stop down to f/8 to get sharp images like a consumer lens.
You won't be disappointed, if you are, then someone shipped you a bad one.