So, I bought my galaxy S6 (64Gb Sapphire Black model) 4 days ago and experienced this issue and figured I would create an account to share my story.
Coming from a flawless Note 4, which has the same type of home button, with a different fingerprint scanner, I noticed the rattling and looseness right away, but didn't think much of it. It was a few days later when I was tapping the fingerprint scanner that I really noticed and remembered a difference when compared to my Note 4 and decided to google search it. Sure enough, I found many threads on the matter including this one. Being the OCD freak that I am, I went to the store and after much debate (obviously, the manager didn't think it was an issue although two tech employees agreed it was) and after comparing with a month old, highly abused display model with a rock solid home button I found that it was, in fact, an issue (although once again, the manager disagreed).
Bottom line, I got it replaced and my new phone has a solid and clicky home button, with a very similar feel to that of my Note. I'm really glad I swapped it out and highly recommend you do so too if you have this issue, it's a very expensive phone after all. I can't say how it will hold up over time, but if it gets back to the rattling, I'm probably going to keep it, since it will be a flaw that came over time in an otherwise flawless piece of hardware and probably will happen to all phones, but rattling right off the bat, not acceptable. I'm pretty optimistic about it though, since I had my Note for 4 months and the button was impeccable.
I hope this helped some of you fellow S6 owners and I thank everyone that shared their experience before me.