Product review (5 stars): Accurate, even if the instructions aren't
Use: I have been using this as the primary gauge on my hermit crab tank for several months (they need 70-80% RH).

The good: Reacts fast
Easy to calibrate, quite accurate (it was spot-on for the "salt test" calibration method)
Has not "saturated" or become otherwise off-calibration or sluggish
Cheaper than more expensive controllers / gauges made "specifically" for terrarium use, more accurate, and better quality

The bad: The instructions can give you a headache, but if you can read a circuit diagram you don't need much of anything else to get it going. The table of modes / settings is pretty straight forward, so even if it reads like a jigsaw puzzle it's not a deal breaker by any means.

One note, if you want to use this in a terrarium like I am doing, you will probably want to shield the cables with something. I didn't think my crabs would find it interesting to play with the wires but I was wrong. Noticed the wire insulation almost stripped off on the red wire for the humidity probe. Wrapped it with electrical tape and hid the wiring better so they don't break it.

So far so good. I will update this review in a few more months with how it holds up.