Based on a previous question What cheap colour calibrators are available for Linux?
One of the answers was ColorHug (from http://www.hughski.com). This seems quite value for money. Has anyone tested it? Good/bad experience with it?
I have one. You're right — it's a good value for the money, and there's basically no catch except that if you're running under Mac or Windows you'll need to know a little more about what you're doing than you might if you just bought one of the big-name devices.
That's because there's only software for Linux. If you are using Linux (any modern distribution), it's pretty much plug in, go to the Color settings, hit the calibrate button, affix the device to the screen, and 20 minutes later you've got a working, active profile. It's very fast, too — much faster than competing devices. (20 minutes is for a high-accuracy profile — you can get a quick one in just a few minutes.)
If you're using a different operating system, you can still use the device, though. It ships with (or you can download) a "live CD" which you can boot and run from without disturbing your existing operating system. Instructions for this are here — in short, it boots to a simple desktop environment from with utilities available, allowing you to create a profile which you can then transfer to your other OS. (It's also easy to make a bootable USB stick from this image. A future version might include that in the device itself, making a super-convenient portable package.
I'd been using my 27" iMac at work (running Linux) uncalibrated, and I was shocked to see the result after calibrating, because it seemed way, way too red. But, after working with it for a while and letting my eyes adjust, I've realized that nope, it's correct — the default was hyper-blue, and the neutral colors look off by contrast. In searching, this appears to be common reaction.
Oh, there is one more caveat. The ColorHug is a colorimeter, not a photospectrometer. If you have a wide-gamut display, you need to have the latter for best results, or at least have a color matrix (which you load into the ColorHug) for your particularly display type prepared by someone who does. There are some pre-existing matrices for such displays for the ColorHug, but not many.
There's also a good review of the ColorHug at Linux Weekly news: https://lwn.net/Articles/499231/.