Firstly, the Pentax is weather resistant (WR), so it's a good match for my Pentax K-5, and I won't worry about it getting wet in the rain.
Secondly, it's smaller and lighter than the Sigma, yet still with great build quality. You can see its size relative to a couple of other common macro lenses here.
Thirdly, the Sigma is prone to flare when shooting directly towards the light, even if it's just a bright grey sky. The Pentax is far better at controlling this (see examples below). This may be because the Sigma is the older non-DG version, so perhaps there's some internal reflections going on between the back of the lens and the sensor. Most of the time it's not an issue, often it can give a nice effect (see the poppy shot below), but when you don't want it, and it rears its ugly head, it really saps the contrast.
Pentax K-5, Sigma EX 105mm f/2.8 macro lens @ f/2.8, 1/60s, +0.5EV, ISO800.
Pentax K-5, Pentax DFA 100mm f/2.8 WR macro lens @ f/2.8, 1/60s, +0.5EV, ISO800.
Pentax K10D, Sigma EX 105mm f/2.8 macro lens @ f/5.6, 1/60s, ISO100, tripod.
Recently, the focus ring on the Sigma has become a bit less smooth too, and it feels like it's lost some lubrication, especially in cold weather. It probably just needs a service, especially as I've had it for 6 years, bought secondhand, and it has had a fair amount of use, but it was a little disconcerting, and focus was not as consistent as I'd like.
Another advantage of using all-Pentax lenses now is that the colour rendition is much more uniform, the Sigma was always cooler and less contrasty, needing different adjustments in Lightroom to get to the results I was after.
Anyway, having used the lens for a while at Colchester Zoo yesterday, I'm pleased with the lens, it's easily as good as the Sigma as far as image quality is concerned, and focusing was much better too.
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