Very good lens for traveling and a good lens for all-around purposes. I use it as my default lens.
For me, OS can deliver up to 3 or 4 stops, and for some reason it works a little when I switch it on (it should start working completely when you half-press the shutter).
Cons include corner sharpness issues in the wide to middle part of the focal range, though the centre of the lens has much better sharpness than that of the non-OS version. However, I've noticed some chromatic aberration with this lens, especially when photographing buildings. Also, being a superzoom, barrel and pincushion distortion also happens. Because of the long barrel, I'd suspect dust gets into the lens easily (as I think mine might!
), and it's not weather-resistant either. Also, if you're a Canon or Sigma user, you still don't get an ultrasonic motor (HSM in Sigma's terms) with the lens, while the Nikon version kinda equals Nikon's own lens by having an HSM motor, so that it is compatible with the D40, D40x and D60. The Sigma costs about $200 less than the Nikon lens. It's well worth the price that it is now, having dropped nearly half it's launch price.
This lens is somewhat of a compromise lens, and the maximum aperture at 200mm is F6.3, in comparison to Nikon and Canon's F5.6. However, in conculsion it is a great lens and well worth it at the price point it is now. It is my default lens for general photography, from Landscape, to Macro, to Telephoto!