My sense is that most - not all - guitars, though they may gain in sound quality and playability over time, have a certain limit to their ability to increase in value. The also have a life cycle and don't last forever. While it is true that the Martin D-28 I bought for $350 (including hard-shell case) in 1967 is probably worth five to seven times that now, 42 years is a long time to wait to icrease your investment. Allowing for inflation, $350 bucks may have represented a greater sacrifice in '67 than $2,000 would today.

I'd much rather invest in Bordeaux or Napa wine futures. Assuming you know your business, great increases are possible within 3-5 years or even less. It's still a bit of a crap shoot, though.