UK's advice above, and comment, is correct - the ultrazoom lenses are not optimal for low light work at max zoom as they are quite slow. But it also depends on what you intend to use it for. There's 'low light' and then there's 'LOW light'. For cruise ship stage shows or Disney shows, they actually are fairly well lit, so any camera capable of producing reasonable quality ISO1600-3200 paired with one of these convenience lenses can actually be just perfect. I've even gotten by back in the day shooting cruise ship shows with a tiny-sensor ultrazoom camera and the stage lighting was bright enough for usable results. And of course, they are unbelievably convenient and excellent travel lenses when you need a one-lens solution for easy carry or inconspicuous travel. While they can't match a dedicated prime focal-length-for-focal-length, neither are they anything to sneeze about for image quality throughout their range. I've managed to get publication-quality results out of an 18-250mm lens at the short AND long end of the telephoto, so they can produce well.
If you need a good telephoto with low light capability for action, sports, dim stages, etc, these lenses won't be the best choice. If you need a good versatile travel zoom that can also handle the occasional well-lit stage or ISO1600-3200 handheld night shot, the 18-250/270mm lenses can be marvelous. Mine's the 18-250 Tamron build, and I'd probably lean towards the Tamron 18-270 PZD if I were shopping for a new one - I have both Sigma and Tamron lenses, but I favor the Tamron's color and image feel in this range.
Oh, and for an idea of the versatility, range and capability of an 18-250/270 lens, here's my gallery of everything i've posted taken with my Sony-badged, Tamron-built 18-250mm lens:
http://www.pbase.com/zackiedawg/sony_sal18250mm_f3563&page=all
Night, day, stage, action, wildlife, landscape, portrait, travel, scenic...I've shot nearly everything with it at some time or another. I have 7 other dedicated lenses for various purposes, but this remains my go-to or default lens.