I've lived with the 18-135 since Dec 2010. It was the go-to lens for me on my travels. I especially found it invaluable when i was in Morocco and in the desert. It was amazing how even when climbing sand dunes and the K-5 with 18-135 hit the sand numerous times as i stumbled, the zoom ring remained smooth in it's operation and the body never skipped a beat even though some dials got a little gritty in operation. The WR is great in such conditions too (apart from wet weather that some have mentioned).
Other have mentioned the other pros of the lens too and the FL is just about perfect.
But yes, i have worried about IQ as it does not compare favourably. The starkest contrast was when i shot the same scene with the 18-135 and a Sigma 10-20 and it was clear the Sigma was way better in IQ.
But looking at the choices around me, i knew that (short of DA* lenses) i had to give up WR if i wanted great IQ with zooms (i shoot primes too but it's not always the most practical in usage and travel), so my (far) heavier kit if i want good zoom now consists of a DA12-24, Sigma 24-70 HSM. I also have a Tamron 70-200 2,8 but its way too heavy to carry. For the tele end when i travel, the DAL 55-300 finds room in my Domke. With this array, i recently went to Prague-Vienna-Budapest and tried to switch lenses between those 2 quickly on-the-fly, as i walked the cities. It does work for the most part and i do get better IQ...
However, as your shoulder starts to ache over the weight and whilst you grapple to pull out/shove in lenses into a floppy shoulder bag, i have to be honest and say i wished i had that light, compact 18-135 with me, more often than i care to admit
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Therefore, whilst the 18-135 may see more time in the dry cabinet hereon, it will still be used fairly often and will not be sold.