Ultimately we all know what this lens is for and who is the target group. As many have mentioned, the convenience of this lens is unbeatable when you travel.
As such we accept some compromise in terms of image quality & barrel distortion(wide-end).
I own this and its almost a "God-send" when I am on my travels. Furthermore, the image quality is not terrible, it's decent enough.
As for more information, there is a heap of information on the web. Visit dpreview, cameralabs, photozone.de, thomas hogans website, kenrockwell etc. for reviews and sample images of the lens. You can find barrel distortion samples in those websites as well.
I have to say it is for now the best "superzoom". The Tamron 18-270 has the benefit of range but a slower focusing mechanism and no internal focusing(which is quite inconvenient if you ask me) I made the mistake of thinking the Tamron 18-270 was better than this till I tried it out and was disappointed with the lack of internal focusing.(If you touch the focus ring while lens is on autofocus it will damage the lens focusing mechanism).
Just to add, if you shoot NEF(RAW) photos, I think most of the distortion can be corrected. I always take photos in NEF(RAW) format.
More importantly; Comparing this lens with Nikkor pro-grade lenses is taking this lens out of its turf. One should be comparing it with Sigma & Tamron 18-XXX range or the like. This lens is in a certain category that only has Sigma 18-200/Tamron 18-200/250/270 & Canon 18-200(if you must compare with this one) as suitable and fair comparisons.
My opinion: This lens is for someone who is looking for an all-in-one lens for Travel. If you want a walkabout lens I think other shorter ranges with higher image quality would suffice. I would recommend this as an all-in-one travel lens; there is simply no better alternative.