I know I use a premium theme I bought, but I'm still not a fan of having to buy them. Basically I decided I didn't care for any of the default themes except for "Night", and that I only sort of liked. So I found that thread with pics and chose 1 to buy, and have used it since. I've never bought an gamer pic though. I got my 360 right after NXE came out, and have used a pic of my avatar the whole time.
Downloadable games I have no problem with. I bought Megaman 9 and Braid and I'm glad. I also bought all but the most recent Gears 2 and Halo 3 map packs.
I guess unless Amazon offers special deals it could be cool (I doubt they will though). Or if I'm trying to squeeze that last penny from my bank account and have to go through Amazon to keep from overbuying points, then I might use it.
Oh I just thought of something. I recall Cobra talking about it being a royal pain in the ass to cancel your Live subscription once MS has your info. Well you can buy prepaid gamecards, but they get your info when you buy points. Not if you use Amazon though.