The 0.45x means that it turns my 18mm into 8mm. It does have a fair amount of distortion, and the aberration is definitely noticeable. BUT - the alternatives are significantly more expensive
- Nikkor 10.5mm f/2.8 fisheye, which will set you back about $600
- Nikkor 10-24 f/3.5-5.6 will lighten your wallet by nearly $800
- Sigma and Tamron 10-20 both cost $450 or so
I paid $8 for mine when I ordered it, and it's currently (March of 2011) on sale for $4.78 on Amazon. For what you get, that's an absolute steal.
Since I got it, I've used it for all sorts of fun projects. I used it exclusively for my first day on thevisualCollective Death Valley Trip.
I also used it almost all day for my recent Free Running shoot with DAFT Free Running.
Focus is still fairly quick - which surprised the heck out of me. Granted, the focus on the 18-55mm (non-VR) isn't very fast to begin with - but this adapter doesn't slow it down much, if at all.
There is a very apparent vignette with this adapter. It gets more and more pronounced the more filters you have, so if you have a UV and a Polarizer, and then you add the superwide, you get a LOT of black corners. I like a healthy vignette in almost all of my images, so that doesn't bother me at all. Because the front element moves when the 18-55 focuses, the degree of the darkening in the corners changes depending on how far your subject is from the camera. Like I said - doesn't bother me.
For what you're getting - I cannot see any reason *not* to buy one of these. Heck, splurge on next-day shipping and it still costs you less than $40. No brainer, especially if your kit lens is just wasting space in your camera bag.
I've got a fair number of fast, beautiful lenses. As such, my kit lens usually stayed in the bag, collecting figurative dust. Since I bought this adapter, I've actually used the 18-55mm a lot and I've been getting great results.
Anyway. That's my two cents. TGIF!