I bought a 32GB 4GB GFX 5k 27" i7 Retina back in October. I was very disappointed for the reasons mentioned in this post and quite a few other reasons too. For example the processor fan would start - just browsing a slide show, and would be on full time running video. My conclusions are that LR and PS were not designed with 5k resolution in mind, so there is quite a bit of catching up to do to meet this demand. The concept was a big leap by Apple, and frankly rather an optimistic one with the available current processors. Basically we are talking about filling a 54 inch screen, and if you have a huge telly in your house, you will realise that this is a massive leap, pixel for pixel.
I was upgrading from a beefy standard resolution late 2012 27" iMac which I was going to use elsewhere, but in the end I decided to return the Retina iMac. Nearly £3k is a lot to pay for what amounted to a performance step backwards, all for the sake of sharpening a perfectly sharp enough image anyway, and the hope of gaining a bit more real estate with some applications. After three weeks struggling with it, it was a huge relief to get back in the old 'Ferrari', my 3TB Fusion iMac.
I might be tempted eventually, but my gut feeling is that the future of super fast 5k is at least two years away, probably three to reach optimum and effortless performance.
So I am very happy now to stick with what I've got. It's super fast and I am in my comfort zone. If you want performance, get a souped up standard resolution 27" iMac and be amazed, rather than disappointed.
