i have noticed that when i am walking around the city, i actually need the wide end of my zoom (in my case that is 18mm) often to take everything in. I also noticed that sometimes, when i want certain details high up in the buildings, i will actually need to pour it right out to 135mm several times. Taking pictures of people will range between 20mm to 60mm most of the time. This confirms that, as a one-lens solution, the 18-135mm is Pentax's most versatile and compact usage.
My gripe is with the IQ not coming close to the Sigma 10-20mm. I want something that can be as useful as the 18-135 and at least have the IQ that matches the Sigma UWA but that's not available. So i have to split that range up somehow.. the thoughts right now is a Tamron 17-50 and a DA*50-135. They will however deliver in other types of photography later though (eg. the 50-135 would be useful in future portrait work) But the problem when you travel is I do expect plenty of lens changes on the go if i bring that combo...
So right now, to keep lens changes to a minimum, i am thinking of getting a 17-70mm and accompany it with a Sigma 10-20 and a 100mm. What do you think of this combo?