I've enjoyed the titanium diaphragms I've put in the tweeters of Quartets, Fortes, and KG4s. I've never changed out the mids.
When implemented properly, I think the metal diaphragms work well. The aluminum used in the Altec drivers to me sounds a bit softer and I do appreciate that. I don't completely care for the titanium in the RF-7II - that actually sounds metallic to me, but could probably be a combination of things.
I wonder if someone did an A/B test on driver material, how many times I could get it right. Part of what we perceive is psychosomatic - your visual perception or prior knowledge will play a part. Floyd Toole had shown this in blind tests. A speaker that LOOKS like it should sound good, had received better ratings when the listener could see it (obviously not always the case, but certainly interesting). I'm guessing the same may happen when we know the material used.