I have this lens and have been very pleased with it so far, got it about a couple of years ago.
The biggest problem I have is that its too wide for certain situations. Its one of those lenses I pack if I'm expecting a close encounter with some large machinery or a ship.
What surprises me the most, however, is that Sigma released another version, but in a constant F3.5 rather than being variable. The 3.5 version was also about £200 more expensive when I bought mine.
Best regards
Gavin