Personally I think the pre-painted stuff looks rather dull and lifeless, and for anything with units you need lots of different poses - I think the GW plastics are wonderful (await a hail of flack) and pray they never move to pre-painted. Also I enjoy painting, even if I rarely do so, no matter how poor its painted its your poorly painted mini rather than some one elses middle of the road.
Finally I think that the pre-paints are the ultimate in gaming dumbing down (just an opinon!) and represent the evil money grabbing methods of selling seen with items like Magic - got to spend lots to get just one pertiuclar thing or give someone else the money.