I wouldn't let a few people's bad experiences with hard drives cause you to cancel your order. I read that thread you linked, and it looked like it was mainly one person with a streak of bad luck with standard HDDs anyway, not SSDs. With any product, there will be people who have problems. Those people will want support and come to forums like these to voice complaints. All the satisfied users out there will be too busy enjoying their new machines to post about it on the forums (and would have little reason to anyway)...
Just be honest with yourself for your reasons for canceling. If it's impatience over the delay, that's understandable. Plenty of people have opted to DIY an SSD upgrade or wait on thunderbolt enclosures with the hopes they are bootable. But personally, while the wait is painful, I'm thoroughly excited to be getting exactly what I wanted - a fully decked out iMac with SSD completely backed by Apple support. If I have any hardware problems, I have 100% confidence that I can exchange for an identical, but problem-free machine. Apple has always been awesome about customer service (in my experience) and I think all the people who have been upgraded to overnight shipping is a testament to that.
I mean, think about it. For many of us (excluding the non-US peeps who have been jerked around a bit more), we knew we were looking at a 4-6 week wait before we clicked "buy" and Apple gave us free overnight shipping for no reason other than to satisfy our impatience. That's pretty awesome IMO.
I just know that I'll ultimately be thrilled with my purchase, even though it'll more than likely be a SATA-II drive and more than likely have SL as opposed to Lion... I'm not building up any unrealistic expectations. I'll be excited just to get what I ordered.