Neat product - It's essentially just another code scanner - the online interface looks pretty friendly... however the reviewer in your link is clear to point out that it won't reset a check engine light, which if I recall correctly is usually the distinction between a $29 scanner and a $69 scanner.
Honestly it is not going to tell you anything earth shattering about your car, and the downside is that it might lead you down an incorrect diagnostic path, but it will be interesting information.
You would be just as well off going to Autozone or another parts store and borrowing their scanner in the parking lot for free, which will give you some more detailed diagnostic info, let you reset a code (so you can see if it comes back or if it was a fluke) and then writing down the error code and doing a google search for "P10xx mycarmodel mycaryear" - that will take you to a bunch of gearhead forums like this where you'll get a better (usually) discussion of what a particular code means for your particular car.
Incidentally, some cars will spit out the check engine code for you on the odometer if you know the right sequence of things to do. (like turning the ign on/off 3 times)
If you really want to buy one, I think you'd be better off spending $50-$80 on ebay, where you'll get a less friendly but more powerful unit. I've gotten by without one for a while but have seriously considered some of them under $40.
I believe this unit at $40 would more or less do all of the things the one in your link will do, plus has the capability to reset codes: http://cgi.ebay.com/ebaymotors/SUPER...motiveQ5fTools
Here's an example of what $110 buys you on ebay which is a MUCH more powerful unit that can read a number of live data points: (no idea of the quality of this unit, just using it as an example) http://cgi.ebay.com/ebaymotors/OBD-2...motiveQ5fTools