Yeah, I never wanted to use it at first. Then I enabled it one day on accident and once I had some of my music running with it I just converted the rest.
Its not so bad, good for when I am going to sleep and que up a bunch of music because it does suck when one song is too quiet to hear and then the next is so loud it kicks you out of sleep.
Even with the gain info in the tags you can just disable replay gain and it wont make use of it, so I can easily go back to not having it interfere with the music.
I have not however found a way to search for files without gain tags to filter them out in any way.