I had the original 55-300 which has recently run down and looks just as if not better. It's very light and compact for a zoom of this magnitude, but loses half an aperture compared to its predecessor. In practice, I found this irrelevant as I didn't take this lens with me for low light shots anyway. What makes a big difference is its incredibly fast focus speed, the fastest of any Pentax lens I've used by a wide margin.
Canon EOS SLR Camera Lens EF 24-105mm f/4 L IS USM
124 Review
New Nikon 18-105mm Vibration Reduction π· Zoom Lens with Auto Focus for Nikon DSLRs
104 Review
π· Nikon AF-S NIKKOR 50mm f/1.4G Lens with Auto Focus: Perfect for Nikon DSLR Cameras
76 Review
Canon EF 50mm f/1.8 II Fixed Lens - Discontinued by Manufacturer
93 Review