Summary: Worth the $16 (after rebate) to get a small improvement in signal.
My house is made of concrete and 3.5 stories high. The antenna improved my reception by "one bar". This was appreciated and did improve my connection reliability or transfer speed. It did not create a huge difference, thus only giving 3 stars.
The most important benefit is having the ability to move the antenna away from the wireless router, based on the cable that is supplied. This gave me the opportunity to maximize the signal strength without having to move my router.
As other people have noted, make sure you know that the antenna signal is orthogonal (perpendicular). Do NOT point the antenna at your computer. This will only weaken the signal.