Originally posted by kthung Avian, I think if your satisfied IQ wise with your current setup, the biggest difference you will see by moving up to the DA*'s is in speed. The two kit lenses (18-55 and 50-200) have a relatively small maximum aperture and if youre shooting in low light, you will have difficulty acheiving fast shutter speeds necessary for certain subjects.
Frankly, that's the only reason I'd consider upgrading my zooms. But since that's an expensive and big/heavy way to get that speed, I'm happier using my cheap M-series primes and my not-very-expensive-either DA40 for speed, and keeping my perfectly adequate DA zooms (18-55 and 50-200) for situations when I feel like the convenience of zoom (and AF) is worth the sacrifice.