I have the wide and tele set for my GSN in the bag, and use them occasionally. While they are not too strong, they have made the difference in a few cases. Focussing is really not that slow if you understand how they work, although they would not be suitable for street shooting. I think the range, with the standard lens, is equal to about 65mm for the telephoto and 38mm for the wide angle.
They do not affect exposure so far as I can tell. The meter cell on the GSN is not through the lens, and I have never done any exposure compensation and my exposure have always been fine. Sharpness has not been a problem either, although I have used smaller aperatures (f8-f11) when using them.
They are fun and neat to have, but not a necessity, IMHO. 😎