I had one and it got a noticeable amount of dust inside the rear element after use outside on a windy day. The gap is quite large between the lens body and and the barrel containing the front element. This barrel is what moves in and out when you focus. I was able to clean the rear element* and decided to sell it in favor of the Sigma. I don't regret this decision at all. The Sigma was roughly half the cost, had much better AF, and with a UV filter on the focusing barrel is contained.
*the rear element is mounted to the lens mount, so removing the four screws holding the lens mount in place will allow you to pivot it out where you can blow off the dust. I did this while wearing a dust mask and un-powdered latex gloves to avoid adding anything worse to the situation. It came out looking like new, but the whole process made me quite nervous.
« Last Edit: February 12, 2013, 04:46:41 PM by bchernicoff »
5D Mk III, Elan 7e, Fuji X-E1, some old rangefinders
Tamron 24-70 2.8 VC, Canon 70-200 f/2.8L IS II, 400mm f/2.8L II, 100mm L IS Macro, Sigma 85mm, 50mm, & 35mm f/1.4's, Rokinon 14mm f/2.8, Canon 2x Extender II, Kenko 1.4x, 430 EX II, Elinchroms