Many shot Kodachrome for reproduction, this is why Ektar is the best modern equivalent, but if you want a film for projection then Velvia 50 in many ways is better than K64 and closer to K25.
Velvia has a truncated color range and over saturates what colors it does represent while Kodachrome has a much smoother and complete range of color, but goes ballistic with reds. I have great KM-25 stock and the only difference I see between it and 64 is slightly finer grain and a tad smoother color range than 64. But they are so darn close that I could have easily passed on 25 and shot even more 64 for price and consistency's sake.
I have shots from New Years 2010 in Times Square that have outrageous color, but not funky and truncated like Velvia would have did, it is really neutral, but spectacular.
I shot it with a Hasselblad XPan and the 30mm aspheric, Nikon F100 and 14-24 2.8, M3 with 50mm 1.4 asph and a M6 + 35 1.4 asph. All did really well with the film, the Xpan having lower contrast that handled high contrast scenes better and the Leica having fantastic contrast and superb color saturation making it outstanding in low light.
I just projected the 24x36 images about 30 minutes ago using a Leica Pradovit RT with a 90mm Super Colorplan Pro lens, there is nothing that can touch Kodachrome, not even close!
The bottom line? In ALOT of situations, there is not going to be much noticeable difference between KM25 and KR64. This is particularly the case with low contrast, sunlit scenes. KR64 does BEAUTIFULLY under such situations. And, in these situations, you would be hard-pressed to see any difference between KR64 and KM25. But under more challenging lighting conditions (especially drab, diffuse light), KM25 generally gives MUCH better results.
I loved Kodachrome 25. Never got friendly with Kodachrome 64.
The difference was big enough for me.
The second one was 64 on a cold-a$$ winter day in waning flat light on the 2nd of this month, I thought it did fairly well.
The last is light that either film would excel in, fresh rain with incredibly even wrap around light ( thank you Times Square ), KR-64 on January 1.
I am starting to use more 25 now that it is 2010, so I bet I will get more of a feel for it as time goes on.
I'm with StorminMatt, my last roll of Kodachrome 64 was on a gray cloudly day, and the slides were just dreary, worse than reality. Nice film with bright light, but you need that light. Since then the only Kodachrome I shot was 200 and 25, and I've used up my supply.
Velvia 50 as a substitute? No way. That's a special-effects film in my book. I liked E100GX (discontinued), I presume E100G with an 81A filter is pretty similar.
Given a good scan, one can get Ektar 100 to look more than a little like Kodachrome.