If you're a visual digital artist waiting to upgrade now may be the time. If you've already decided Apple is your OS of choice, and you'd like some more power being your editing AND you'd like to see what having more pixels in your display can do to your workflow then Apple's quietly made HUGE change may be a windfall for you.
I've been editing on 2800 pixels across on a 15" MBP retina screen for 3 years. It's been fine until I needed to do newspaper layouts in inDesign, wedding albums in PS, and see the color across multiple faces at once in LR. For that, the 15" screen just wasn't going to cut it. I've been looking at getting a second screen to add to my laptop to give me more screen real estate. The problem is the cost is enormous for not a lot of pixels. So Apple does this crazy thing and puts a monitor into their new iMac that if you both it along would cost more then the whole desktop all in one package. It just seemed like a no brainer to me. So I just purchased one. Then yesterday I found out with El Capitan Apple quietly introduced 10 bit display architecture into their OS. Then I found out today that the new iMac supports it. So for the first time ever I'm now editing in 10 bit. Also, the new iMac expanding the color space to a larger space then previously, about 20% more colors. That with the insane # of pixels makes now the time to get one of these beasts. Hopefully in the next few months LR and Ps will be upgraded to support 10 bit on El Capitan. When the retina screens first came out Adobe was one of the first to support it.http://appleinsider.com/articles/15/10/30/new-4k-5k-imacs-support-10-bit-screen-color-for-improved-image-accuracy