Comments about Tamron Tamron SP AF 17-50mm f/2.8 XR DI-II LD Aspherical (IF) Standard Zoom Lens for Pentax, 6 Year USA Warranty:

Such an upgrade to any manufacturer's "kit" lens - the 17mm covers most wide angle situations you can run into, while @ 50mm provides a fast f/2.8 aperture normal lens for capturing portraits.

Studio and "strobist" users will appreciate the constant aperture at all focal lengths.

Only caveat so far is some slight hunting of the AF in low light (comparing this to the super-fast 40mm f/2.8) but it's more a matter of learning the quirks of this lens.