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On the outside, the Retina iMac remains essentially identical to the regular 27-inch iMac. It makes it kind of an astonishing thing to have on you desk in one tidy package. Apple's best desktop yet? Probably. Surely, even.
We liked
The iMac 5K with Retina has one of the best displays that money can buy. The modern computer experience (particularly so on Yosemite) is hugely visual, and having a super-sharp Retina display just makes it a joy to use. It's one of the best all-round computers on the market for photography enthusiasts, designers and other multimedia editors seeking a compact all-in-one with a vibrant and colour-packed display.
And it's all wrapped up in a relatively compact package that's simple to set up and won't take up much room on your desk. There's plenty of ports (including the highly useful Thunderbolt 2), and the ability to easily upgrade RAM, hard drive, CPU and other components (albeit with some difficulty) will be welcome by anyone worried about the lifespan of such a wallet-intimidating machine.
We disliked
The obvious downside to the iMac is that it's expensive, but when you consider the cost of a 5K monitor and an equivalent-specced PC, there's not much in it. Dropping down to a 4K monitor, on the other hand, opens up combinations that will cost a lot less than the Retina imac.
The absence of Target Display Mode is a big loss, especially when previous iMac models permitted the connection of secondary devices. It wouldn't be surprising if some of the pros it should target will be hoping for an equivalent Cinema Display to connect to a Mac Pro.
iMacs have never been famed for gaming, and gamers plumping for a Retina iMac will need to temper expectations. Though it depends on the game, more recent titles simply won't run at acceptable frame rates on the entry-level's mobile GPU. Opting for the Radeon with 4GB of video memory may help with this, but without benchmarking that machine it's impossible to say for sure.
Final verdict
The iMac with Retina 5K display is the best-looking all-in-one on the planet, and stands tall as the best way to experience OS X Yosemite in its high-ppi splendour. But be warned: once you've clasped eyes on its crispness you'll find it difficult to go back to what you were using before. You may have to save up for months on end to afford one, but the longevity afforded by the display and means that it will serve you well for some time.