You lose both f-stops and quality.
The f-stops is simple. A 2x adds 2 stops and a 1.4 adds 1 stop (rounded). This is due to the fact that you are changing the focal length while keeping the lense diameter the same. F-stop is Focal length/diameter. So if you have a F/2 lense it is now a f/2.8 with a 1.4x and a f/4 with a 2x.
Here is the modern f-stops:
f/0.7, f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32, f/45, f/64, f/90, f/128
So if you have a zoom which is f/5.6 on the long end (common on the bundle lenses) it would be a f/11 with the 2x at the long end!
As far as quality, you add glass surfaces so it will hurt some. A 200mm lens usually will outperform a 100mm with a 2x.
Any way I have used a 2x with my Pentax and Zenit cameras and I am happy with it. I actually have used a 3x with a 300mm prime before too and was happy with the results.