With the dSLR (K-7, K-x), the typical all-in-one zoom lens is the 18-200 or 18-250. These lenses are all-rounders that covers a wide range from 18 mm (wide angle) to 250mm.
There has been some recent pertinent discussions at DPReview as well as Pentaxforums.com that I summarise herein. There are basically 3 generations of all-in-one zoom lenses for Pentax (K-mount): (a) the Sigma and Pentax 18-200mm, (b) the Tamron/Pentax 18-250mm, and (c) the Sigma 18-250mm OS HSM.
The Sigma and Pentax 18-200mm lenses are older lenses that were superseeded by the Tamron/Pentax 18-250mm. The Tamron 18-250mm and the re-badged Pentax 18-250mm are definitely better lenses than the older Sigma 18-200mm or Pentax 18-200mm.
A few recent threads discussed some comparison between the Tamron 18-250mm and Pentax 18-250mm: eg,
http://forums.dpreview.com/...forums...ssage=33945288. For the latter (Pentax 18-250mm), the Pentax K-7 and K-x have the lens correction implemented and you can select some distortion correction to this lens :
http://forums.dpreview.com/...forums...ssage=34042860. A lot of pentaxians would recommend the Pentax/Tamron 18-250mm lens. The DA 18-250mm is an excellent walk-around lens and one of the best in its category. Full stop; it is not a fast prime and should not be compared with prime lenses.
There is less information on the newer and more expensive Sigma 18-250mm DC OS HSM. The reviews are positive but (a) there have been some forum discussion on reliability & maintenance issues of the HSM technology and (b) the optical stabilisation OS is redundant with the Pentax image stabilisation IS.
I use a Pentax 18-250mm with my Pentax K-7. I am happy with the Pentax 18-250mm in terms of practicality and ease to use in the field (no need to change lens), as well its image quality. I would recommend it. I like the compact shape and I am able to get the K-7 with the 18-250mm in a small top-loading bag (Lowepro Top load zoom 1) that is handy to carry.
Hope that the advice will help...