Personally I wouldn't consider using it unless the sun is out as I find that it is difficult to get the light okay in the slides with it being only a 64asa speed film so I keep it for sunny only shots and have been using it on the more regular aircraft that I shoot.
Comparing it to Fuji's Sensia 100asa you can see that there is less grain in the k64 slides so they look very good if you get the conditions correct and this is why I have continued to use it for occasional shots.
In my personal opinion there are two drawbacks with K64 slides and that is the slide holders that they use and there can be problems with their processing.
Getting Them Processed.
I like to post off some slides each month to a magazine and this time lag is an obvious drawback so it is one reason why I prefer to use Fuji who are much faster as they process them in the U.K.
I have had a number of problems with Kodachrome processing and have had slides back with chemical splashes, quite a few badly mounted in the slide holder and a handful badly damaged with a crease right through the middle of the slides.
In January 2002 I returned 13 slides with 10 of them having chemical splashes and the other three ruined as the slides were badly creased so after a wait of nearly a month I received a letter of appology which said that they couldn't do anything to wash off the chemical splashes plus a single roll of film as "compensation", I can't say that I was impressed with that.
In April 2002 I posted off two rolls of K64 a week apart and they lost them both, after various communications through the Post I finally received a letter 3 months later confirming that they were lost and the letter appologised for this and said that two rolls of film were enclosed to cover the loss of my films but in the jiffy bag there was only one roll, so having lost them they couldn't get that right either.
I wrote off again pointing out the error of only one roll being sent and a week later the other roll arrived, I had thought that they would have put in maybe a couple more but no they didn't so the upshot of this from my experience is that they can be slap dash and make mistakes. Considering that about 90% of the time I use Fuji film these errors have happened too often for my liking.
Slide Holders.
I am quite fussy in getting the slides looking level and as full frame as possible so I will often open the holder up to reposition them, if you are trying to get your subject to fill nearly all of the slide then it is inevitable with moving ones that you can nip the nose of it or leave a bigger gap at the back so I think that it is important to be able to open the slide holder, reposition the slide and then close it again and the Kodachrome ones just won't do that.
Even if they did the other problem is that the slide is held in place by just a very small piece of sticky tape in one corner only and when the slide comes off you can't get it back on again so I am less than impressed with the holders and have had to use a good number of my spare Fuji ones when sorting through the K64 slides.
It might sound a bit fussy talking about repositioning the slides to get them straight and in the middle of the frame but the way I see is that with the amount of time and travelling you put in to get them (never mind the cost of foreign trips) then you should have them looking as they should and not sloping one way or the other or having a big gap at one side.
Colour difference.
Summing Up.