Ken,
As Lynn said, it depends
I personally have following 4 lenses that cover most of my needs - Distagon 28mm f/2.8, Planar 50mm f/1.4, Sonnar 85mm f/2.8 and Sonnar 135mm f/2.8. I also need Planar 85mm f/1.4 and Distagon 35mm f/1.4, but I will add these later
Marc,
The main problem with IS is the fact that it is accomplished via a group of additional elements positioned inside the lens. This group is there regardless of whether you use IS or not. IS is accomplished by moving this group "correcting" your handshake. The problem with it is that:
a) it requires complete redesign of a specific lens to make it an "IS" lens.
Case in point - 70-200mm f/2.8 L "IS" has 23 (!) elements in 18 groups (thus 36 air-to-glass surfaces)
70-200mm f/2.8 L has 18 elements in 15 groups (30 air-to-glass surfaces) and 160 grams lighter
b) with these extra air-to-glass surfaces light loss becomes more significant and it's more prone to flare. See point "a".
c) more elements means more complex manufacturing, harder to achieve tolerances when assembling, mounting and centering lenses and different barrel from a regular lens
d) mechanism that moves these "IS" group lenses has to be very precise, which is easier said than done. It also depends on how accurate these gyro sensors are. Thus - shifts in focus and various aberrations even when the IS is turned off are possible, since that group moves up or down perpendicular to the optical axis - meaning there is a room for it in the barrel.
Nikon didn't start making VR lenses until relatively recently for pretty much the same reasons - they saw more cons than they saw pros. Only demand for these kind of lenses forced them to start making them.
Mike.