I have and use one on my 60 gallon tank. Just has a suction cup and sticks to the inside of the glass. I keep mine about halfway down on the left side of the tank. As I'm sure you've probably read over and over, nothing will be as accurate as a testing kit like API; but I wouldn't disregard this item.
It only measures the toxic ammonia (NH3), not the total ammonia (NH3+NH4) so will not match up with the test kit which tests the both NH3 & NH4. If nothing else, it is a good first warning for an ammonia spike. It lasts either 3, 6, or 12 months, I forget what it said on the package (Will check when I get home, but I think 12 months because that's what it says on your link). And I know mine works because it quickly changed from yellow to green after an untreated water change because my tap water has at least 1ppm ammonia in it.
That's my opinion. I'm still relatively new to aquariums (few months) but it's relatively cheap and I don't see a downside to having it. Just hide it in the back corner if you think it's an eye sore.
/www.waterzoo.co.uk/ProductView.aspx?ProductID=294&Mode=Existing
