I've been quite pleased with the performance on my 18-55DX on my D50. However, as I've found that I've used Manual Focus more and more recently, I've seriously debated moving to a true AF-S lens for the instant manual override. I tried a friend's 18-70mm and was quite pleased with the rapid focus, larger aperture at the tight end and M/A mode. The extra reach didn't hurt either. My question is this: Would it be worth it to pick up a used 18-70 and sell my 18-55, or should I just start saving up towards an 18-200VR? Most reviews of the two I read would indicate that 18-55 to 18-70 is largely a sideways step - their performance is supposed to be quite similar. Is it worth the switch?