I really like the 45 FL. I tried walking around with my 40 - 150 set at 75 and I felt uncomfortable taking pictures. Despite the really wonderful, sharp pictures that 75 owners have posted, the FL is not for me.
They should be compliments to each other, not substitutes. It all depends on what your needs are. The 45 is definitely more versatile to have if you don't shoot portraits often. The 75mm is a beautiful lens but spending $900 on a lens for casual snaps on occasion is silly.
I have both in my bag. I'm not a huge fan of the 45mm, never have been. But I end up needing the FL very often for what I shoot. The 75mm is on my camera a lot of the time, which ends up forcing you to work with it and always be stepping back a bit further. To me there are some special qualities of the 75mm that the 45mm just doesn't have. I take far more portraits with the 75mm or up close with the 17/25. Everyone has their own style so it's really a decision left to you.