Just to take up a point raised by @frogoutofwater and @emalvick, I have both a superzoom (Tamron 18-250) and the Pentax 55-300. I can add something about the comparison.
No question that overall the 55-300 produces better images. It is about a stop faster across the range (at 200mm it is f4.5, and 250mm is f5.6, which is not bad for a lightweight zoom). Also, because it is not an internal focus lens, it has greater magnification at any given focal length than the 18-250 (other than when focused to infinity). So the advantage in reach is much greater than 250mm v 300mm: what the Tamron can reach at 250mm, the 55-300 could often reach at 200mm. For wildlife that is a big advantage.
But there is more to the comparison than this. If you framed the subject in the same way (which - just guessing - might mean the 18-250 at 200mm and the 55-300 at say 150mm for a subject 6 metres or 20 feet away), and stopped each down to say f8, you would need to be a dedicated pixel peeper to tell any difference in resolution. After correcting in PP, I would have to look at the EXIF info to see which lens it was.
If you are a dedicated pixel peeper, have a look in these reviews and you will see that there is not much in it for centre sharpness stopped down:
Tamron AF 18-250mm f/3.5-6.3 Di II LD Aspherical [IF] macro (Canon) - Review / Test Report Pentax SMC DA 55-300mm f/4-5.8 ED - Review / Test Report
At the wide end, particularly wider than say 70mm, I would actually prefer the Tamron even though it is a little slower (f4.5 v f4 at 55mm, and f5 v f4 from 70mm-100mm) - I think the colours are better, and I prefer the bokeh (although neither is great for this). The variation in individual copies might account for more of the difference than inherent design. Maximum magnification is similar (about 1:3.5) but the Tamron's minimum focusing distance (450mm v 1400mm) makes it much more usable. If you don't have the WR version of the 55-300, it doesn't have any advantage in robustness; like Earl, I have found the 18-250 very robust. The 55-300 is about the same weight but physically a bit longer - it won't quite fit on the camera in my snout bag.
I have never used the 18-135, so I can't compare it to the 18-250. But there are plenty of user reviews and comparisons out there and the consensus seems to be that the 18-135 is a bit better.
My view is that,
as a two-lens solution, the combination that frogoutofwater suggests (18-135 + 55-300, especially with the WR version) makes a lot of sense. The overlap in focal lengths is an advantage, and their strengths are complementary (the 18-135 at its wider end and the 55-300 at its longer end). But the question was, is a
one-lens travel kit worthwhile? To which I would say yes - get the 18-135 if you don't need the extra reach (for wildlife etc) and a good superzoom if you do. Of course there are compromises, but particularly for outdoor use what is lost in IQ is more than compensated for by the gain in versatility.
Similar Threads 
