Bard and Dave,
Please remember i am talking Canon here, i have not used one on a Pentax, however i can't see why it would not be the same.
The Canon has both 2x and 1.4x and can be switched to either while looking through the finder.
Once you have got used to it and found the sweet spot, while looking through it, "so to speak" they are great.
I wouldn't do without mine.
Leon