I bought this lens on the recommendation of a friend, a professional photographer with some serious talent.
The majority of my photography revolved around taking pictures of bands in live music venues, so I had a major need for a lens that would allow me to capture decent pictures in low light and at close quarters - my friend recommended this one to me.
I've been so happy with this lens it's almost disgusting. I've managed to to get some unbelievable band pictures in virtually no light and even stood in front of the audience, not 10 feet from the band, i've still got enough of an angle to get a full stage shot.
The F/Stop on this lens goes right down to 2.8, which lets you swallow up all the light in the room. It doesn't take a hugely long shutter speed to grab enough light for a decent shot, which is turn means you can get some relatively 'live' low light shooting without worrying about your subject blurring.
The focus on this lens is fast and accurate, and the mechanics are pretty much silent. I didn't appreciate these features fully until I bought a couple of cheaper lens, and bands stopped playing when my autofocus whirred along.
Overall, this lens feels good on the camera - it's quite weighty, but not in the grand scheme of things and ultimately it makes you feel more confident about the build quality of the lens. It also helps justify the price tag, although with the results you'll get from it, no additional justification is needed.
If you're looking for a close range, lowlight lens, this is the one you want - any lower a focal range, and you'll get warping around the edges of your picture (you'll not be far off a fish-eye effect).
About the only time you can go wrong with this lens is when you get carried away and presume this lens can do everything...as soon as you try taking a landscape shot, you'll realise this is not a all encapsulating lens. But then, halfway through your disappointing trip taking pictures in the Lake District, you'll visit a castle and be amazed as the pictures you get in the confinded spaces.
Basically, this lens rocks at close quarters AND low light...either seperately or together. It doesn't rock at taking pictures of mountains. Does that make sense?