Product review (4 stars): Works great, PC required
I wanted to keep track of the temp & humidity in my garage prior to installing a exhaust fan. The unit is well built and comes with several useful accessories. From the front panel you can see current/max/min temp & humidity.  You need to connect it to a PC via USB to set the unit's date, time, and logging options.  The software installed fine on my Win 7 64-bit PC. I downloaded the latest version from the Extech website. The software works surprisingly well.  I was able to quickly set up a logging schedule, set the time on the unit, and switch to Fahrenheit.  Many logging options are available, from 1 sample per second (max 4 hours, 32 minutes) to one sample every 24 hours, and everything in between. I used one sample per minute which will last 11 days 8 hours.  You can have the unit stop logging after it collected the max # of sample, or overwrite the oldest data.  It records temp & humidity simultaneously. After a week of of logging, I connected the unit to my PC and was able to download the data quickly and display it in graph or spreadsheet form. If you export the data to a file, it is a tab separated file, and can be opened in a spreadsheet.  Each timestamped (m/d/y h:m:s) temp & RH reading will be in the file.  The graphic display can't be saved by the program, but you can screen capture it.  Data can also be stored in a propriety format for reading into the software later.

On the not-so-great side, You can't do much from the front panel, like change from Celsius (factory default) to Fahrenheit or setup logging, or view previous logs.  User manual is sparse (-1 star) and a little helpful. If you're not handy with a PC, this unit may not be for you. The front panel display is a bit confusing, there's some data there I don't understand. No backlight. The battery is one of those expensive non-rechargeable camera batteries, a standard 9V or AA, AA would have been nicer. There are two tiny (easy to lose) screws holding the battery cover in place, but the screws go into a metal sockets, not plastic - thank you Extech for that. The battery cover also looks to be environmentally sealed.

Overall, I'm very happy with my purchase and recommend the RHT20.