I fly Delta several times a month, have an MBP and use GoGo and it has always worked well for me. It's not as fast as my internet at home, but it still works fine. Not sure what's going on in your situation, but that's unfortunate. Hopefully it was just something specific to those two flights.
I don't know much about the service itself (specifics about the internet connection), but it is quite a few people using the same (router? I guess? not sure what's going on up there), and I know that when my old house's router wasn't password protected, other people would leach our internet and slow it down quite a bit. I don't know if you just have very high expectations or what, but I am used to very fast internet and the GoGo connection seems pretty quick to me.