Yeah, I would watch how you spend the first few months; however you're entering the awesome world of manufactured spending!!
If you have another card, I would suggest doing this for your loans that do not accept credit card, as this is a great way of getting points for what you spend. There is a great forum on Flyer Talk that can get you way more information, but I use Blue Bird and Vanilla Reloads, which go onto my My Vanilla Debit.
I spend about $2k/month on giftcards and swap them between the cards I have used. I have used my AMEX before to buy a reloadable giftcard (only $500 worth), and no harm was done, but it would not be something I would do monthly just because I have a small limit.
I think being practical is the best answer. Some people do go overboard and solely use the system for their sign-up bonuses. However, if you have legitimate reasons for buying the giftcards, there is no harm or shame. IE I have no problem explaining to Chase or AMEX that my apartment complex/utiltity company does not accept CC, so I use Blue Bird (which is through AMEX) to pay with them.
