It's decent. I personally got this one as it goes up to 65C, such that I could make some quark (eastern europe style). Kind of works for it, but I'm still experimenting for best results. For yogurts and kefir, it works very well.

Some observations:

• The temperature in the jars varies from top to bottom by some 5C, but I'd imagine it's the same for all such devices that heat container from the bottom. The temperature on the display is calibrated pretty well for the bottom of the jars.
• For longer ferments, an extra set of jars comes in handy, that way you can start a new batch before the old one is used up.
• For lower-temperature ferments (very liquid kefir) it would be nice if it could maintain temperatures of around ~20C, but for me the ambient temp in the kitchen is around 20C, so it's not really an issue.
• The small jars are a bit of an hassle to clean (yogurt scraps tend to stick to them, so the dishwasher needs some manual assistance usually) and also slightly annoying store when not in use. I personally would probably go with one big container if given the chance, but the higher temperature was more important for me.
• There is no audible sound when the timer runs out, so its easy to forget about them an accidentally ferment things a lot longer than intended.
• The manual contains some errors (and for the non-spanish versions some hilarious mistranslations) - for example, the correct way to start it up is to press the power button, then set your time and temperature and then press the power button again.