Was looking at these over a year ago, and have on numerous times considered getting one. I then found a great review (which i cannot find right now) that gave an excellent run down of the pros, and cons.

The main one that caught my eye was that the claim of 3, 3.5, & 4 times mag is not accurate. If you have a 25 power scope it will not turn it into 75 power, but more along the lines of 32-33 power. So the 3, 3.5, and 4 times on it really give 30%, 35%, and 40% more mag.

Not sure what models they work on now, but at the time it was limited to NF, and a few other brands and then only one certain models within that brand.