Oh please. Maybe you got a dud, maybe your expectations are unreasonable, maybe you don't know how to use the lens, who knows.
A few things - Canon's prosumer DSLRs are NOT reknowned for fast AF, especially the cheaper ones. The 450D does NOT have fast AF. Nor accurate. I've been using Canon's for 20 years, I have a 1D Mark IIn and D60 (great grand daddy of your 450D) and there is a marked difference in performance between the 2 cameras. The next time you're in Brissie, PM me and I'll be only too happy to meet up and let you have a spin with your lens on my Mark IIn. That'll sort the boys from the girls out I suspect, and might make you feel a bit better about the lens.
A few things - the lens is a slow lens - min. f stop is f4.5. That doesn't let much light in, which means the AF sensor has less to work with. There's a reason why pros pay for super fast pro lenses! You pays for what you get.
Also - Sigma makes some mighty fine lenses. I own a Sigma 150mm f2.8 EX APO Macro lens, and it's a winner. Equals Canon's much more expensive 180mm f3.5 L macro lens imho. Do a bit of reading on the net and you'll find MANY happy Sigma users. My macro mate LordV (Google him) uses a Sigma macro lens (as well as a dedicated 1x-5x Canon MPE-65 unit) and his Sigma images are wonderful. Hell, all his images are.
One final thing - know your consumer rights. If you are not satisfied with the unit, the dealer MUST address it, not fob you off. Get their postal address, send them a letter via registered post (signature required), demanding a response within 14 days or you'll take it to your local Fair Trading.
http://www.accc.gov.au/content/index.phtml/itemId/3863
Also, for your interest, a review on the lens you bought:
http://www.popphoto.com/Reviews/Lens...-HSM-Lens-Test
I don't mean to belittle you, but I find most people who are new to photography don't know enough to know what's a reasonable expectation, and what isn't.
Dave