I personally prefer to do it that way--you can create a more convincing review that's more likely to make sales. It's not always possible or practical, though; for example, would you break up with your significant other just to test a product for getting your ex back? ;-) In cases like that, or if the product is expensive, it's usually best just to use the vendor's affiliate resources instead.
But I think the biggest deciding factor in this, goes back to the site as a whole and all of the other posts. Are the genuine? Is the blogger constantly trying to push products? I’d like to think I’ve been doing this long enough that my audience knows I’m not out to make a quick buck – and I think even relatively new bloggers can prove this based on their other content.