Nick, in general, the longer the zoom range the greater the drop in ultimate image quality and this was certainly the case when I went for an all-in-one, single lens solution several years ago. Of course, I was then on a fairly tight budget so went for a Tamron 28-300 and, while it was far more convenient than my 28-80 and 70-300 2 lens kit, at anything much above 7.5 x 5 inch prints, you could spot the shortcomings from a mile away (soft, severe distortion at the wide end, chromatic abberations) and while the buyer was happy with it (since he shot lots of rallying, he didn't want to be swapping lenses and his end uses were web use and small prints), I was happy to go back to a 2 lens solution.
However, lens design has leapt and bounded ahead in the intervening years so the current crop of similar lenses should be streets ahead of that old Tamron!
I should add that my current solution to the "problems" that a full SLR bag brings (bulk and weight as well as the hassle of lens swapping) is a bridge superzoom (Fuji HS-30) and a decent pocket compact (another Fuji, an XF-1). Ultimate IQ isn't up to that from a D800 and interchangeable lenses but it's so much lighter and less hassle! I'll be keeping the full DSLR kit but for most uses I now take the Fujis.