I had the original 15" Retina MBP from 2012 (Apple Certified refurb), which I soon regretted buying because just a few months later the Late 2013 MBPs appeared with greatly improved specs ( as often happens with the 2nd edition of Apple products).Thanks for the info! Which model did you have before, and what's the difference in power in terms of using it? Also, what do you expect the depreciation to be? i.e. 10% per year.
Upgrading to the 2015 model I immediately noticed the much smoother graphics on the retina display. Speaking of the display, it's better looking, and easier on the eyes, with richer, deeper blacks. The computer feels faster overall, thanks to the 4x speed. I love the new Force touch trackpad, and could never go back to the old style, which now feels clunky to me. Battery life is a big improvement: I can now get through the whole day without worrying.
As for depreciation, expect 10% - 15%, but only for the first year. The percentage tends to get larger per year after that. If you are in New York City, where there's much more competition from people selling their Macs, the value can drop even faster.
The worst time to sell is right after a new model launch -- that's when the market is flooded. It's better to wait a few weeks until all the action is died down. However, the best scenario is selling on a solid rumor before the launch. This way you get a really good price, and will only have to be without your Mac for a few days.