A common criticism is that there isn't the same range of lenses available for Pentax as there is for Canon/Nikon, and there isn't. There is still a perfectly adequate range available though.
And............. if a shop is having an "everything must be sold" clearout sale, or even an online retailer is trying to shift some lenses that are about to be discontinued because the new DG models (it makes how much difference?) have just been launched, the Canon/Nikon stuff goes in an instant for probably about £10-20 less than the normal price. Pentax/Olympus/KM fit stuff sits there and gathers tumbleweed. The price keeps dropping, and dropping, until you pick up a steal of a bargain.
Sometimes, other WPF members even give you their perfectly good lenses because they have no use for them any more.
(I gave a donation to the charity of Chris' choice as he wouldn't take money for it).
There are hidden benefits for using minority brands.
Duncan