A comprehensive rundown of close to everything you need to know about the Sigma 18-300mm f/3.5-6.3 DC MACRO OS HSM Contemporary Lens for Nikon F Mount, in particular as it is used in a Nikon D5100 body:

[0:45] Ergonomics
[2:02] Vignetting and Cropping
[3:48] Optical Stabilization
[5:32] Weight
[6:12] Minimum Focus Distance
[8:30] Sample Shots

High level summary: great lens, but crops in the left corners. Take a look at the sample shots to see what I mean.