When I first bought my KM5D, I also bought the KM 18-200 (a rebranded Tamron, and the equivalent of the current Sony) thinking it would do for what I wanted, and initially, I was pleased. Then I started looking critically at my photos, and wasn't so pleased.

The 18-200 has significant geometric distortion at the wide end, significant chromatic aberration at the long end, and is soft through out its zoom range. I have since replaced it with multiple lenses with less ambitious zoom ranges, and am glad I did.

The Tamron 18-250 is apparently a much improved superzoom lens, arguably the best of its kind, but it still suffers from the same problems, albeit to a lesser degree.

If you do decide to go the superzoom route, the Sony 18-250 is a rebranded Tamron, but with improved autofocus gearing for Sony dSLRs, so that's probably a better choice.