Lots of good advice in these posts; Mike's sums it up pretty well. Re the collectible value, while there's no disputing the worth of older/rare material, it's always a little dangerous to extrapolate trends into the future on a "straight-line" basis (not that many of us don't do this at work, however!) Price comes from demand, and as fewer people (likely) shoot and collect slides, there won't be as many people searching for them. Also, unlike the 50s/60s/70s, there are lots of people shooting in more recent times, and in many cases, lots of slides out there. Many people will be happy to simply view aircraft on sites like airliners.net; this option didn't used to exist, so that if you wanted to be able to see a particular subject when you needed, such as for reference. you had to have your own copy, and slides are a reasonable way to accomplish this. After 30-plus years of slides, mostly Kodachrome, with a recent switch to Provia (just got the new 400F to try out!) it's sad in some ways to see slides going out of favor, but likely inevitable (although black and white still has its adherents, and there will be some shooting slides as long as the medium is available). Still, at the moment, it's probably the best way to get a high-quality original, positive image, and they're relatively easy to store and retrieve. Good quality/rare ones will continue to have value for a long time to come, but this will become less and less relevant to the average airliner photographer.