I know it's not exactly a full and forensic review, but I did try a Sigma 18-300 in Gatwick airport when I had an hour to kill once, because I couldn't spend another minute sitting in Starbucks. The lens was genuinely terrible, but then it was also ridiculously cheap - you get what you pay for I suppose.
I looked briefly at the Nikon 18-300 not too long ago as a "touring" lens when I only have room to take one lens with me. However I was put off by the extra weight over the 18-200 which is what I bought in the end.
Like others have said though, the 18-300's tend to be a bit of a "jack of all trades, master of none". I suppose the same could be said of 18-200, but I don't find what comes out of it too bad actually. I guess it depends how closely one looks...