It does work 5 days early, but it depends on when you O'd, and how quickly you produce HCG. It's just a very sensitive HPT, but the results will vary from person to person.
I think somewhere around 18% of people will test positive that early. The bad news is, it also increases your chances of catching a chemical pregnancy (where the egg doesn't implant) that you would not have known about otherwise.
Personally, if you can wait, I would. I'm also kind of cheap since I've spent so much money on these tests. Plus, I got my BFP w/ a $ store test before my $10 digital Clear Blue showed positive.
So, IMO, it's not worth the extra money. If you want to test, go for it!

BUT, if you get a negative and don't get your AF, make sure to test again, because testing that early shouldn't be taken as a guarentee.
Good luck!!!
