I had a Tamron 18-250 as my only lens for a number of years. It's the same as the Pentax DA 18-250, and probably fairly typical of superzooms (ie lenses covering wide-telephoto focal lengths, with a focal length range of 10x or more). The Tamron was regarded as a significant leap at the time of its release in about 2006.
I now use the DA 18-135 as my main lens for a hike. If wildlife is a possibility I will take the 55-300 and swap them or take two bodies. The image quality from this combination is significantly better than from the 18-250, but at the cost of extra weight and bulk and more inconvenience. That's the trade-off.
The superzoom did suffer a lot from focus breathing, so the actual magnification for birds etc was a lot less in practice than its maximum focal length (250mm) would suggest. I understand the same is true of the DA 18-270, which is a rebadged version of Tamron's successor to the 18-250. Not sure about the Sigma 18-300 in this regard.
Despite its limitations, I got plenty of worthwhile shots with the superzoom, even though in those days I relied on jpgs only. See examples here (post #6):
Opinions and experience with the 18-250mm? - PentaxForums.com
The convenience and weight-saving of a superzoom makes it very handy as a one-lens travel option.
If you shoot in RAW, some of the shortcomings (distortion, vignetting, CA etc) that are characteristic of these lenses can be worked on in post-processing.
FWIW, Lenstip, which is usually unimpressed with superzooms, was reasonably kind about the Sigma 18-300:
http://www.lenstip.com/427.1-Lens_review-Sigma_C_18-300_mm_f_3.5-6.3_DC_MACR...roduction.html
It was less impressed with the Tamron 18-270:
http://www.lenstip.com/251.1-Lens_review-Tamron_AF_18-270_mm_f_3.5-6.3_Di_II...IF)_MACRO.html
Don't give too much weight to these reviews, but they add something to the information mix.