I think the Tamron 17-50 f/2.8 is a great lens.
When I bought my KM5D, I got the 18-200 f/3.5-6.3. It did ok for some things, but the more I used it, the more I reallized what a piece of crap it was. I got ahold of a number of used Minolta AF lenses and never really used the 18-200 again, but didn't have anything that could go as wide, and as I've always been an available light photographer (dating back to my SRT-101 & -202), I knew the kit lens wouldn't do it for me. I spent a lot of time looking around and settled on the Tamron 17-50. I'm very pleased with it. It's brighter, sharper, and has less geometric distortion than the 18-200 (which wouldn't be hard.)
I posted First shots with Tamron 17-50 f/2.8 on my KM5D
a while ago after I first tried it. It contains some sample shots.