I use them occaisionally, depending what your target keywords are will determine the cost. For a competitive key word/phrase expect to pay arround £1.00 per click. Google uses a quality score, so sometimes it is not possible to outbid another ad (so figure higher on the page) depending on how good / poor your site is will determine the posistion of the ad on the page.
If the quallity score isn't high enough, the ad may never show or be burried on 'see more ads'
It can get expensive...very expensive if not careful. Set a daily budget maximum. Make sure your keywords are very targeted.....