As far as being an investment, I think it's a little early to tell - they are such a new product category ...however, they are increasing in popularity mainly because
1) the cost of housing is high
2) hotel costs are equally high
3) people are increasingly traveling the world and having 2nd and 3rd homes but don't want the hassle of "whole" ownership.
Time will only tell unfortunately. If you are worried, make sure you buy something truly unique though - something that will stand out when you try to sell again - 1) views 2) location 3) more views 4) better location... etc.
Hope this helps!