I was browsing a Gamer's Paradise in Chicago and they have 50% off on red-stickered games. One of them was Odin Sphere, so I grabbed myself a copy. Thought you might want to know if you haven't gotten one yet!
1. I played GGX way back then and liked it. I would purchase it just for that.
2. Personally it's a pretty easy game to pick up. Just like other fighting games
3. Compared to a game like Street Fighter, GGXXAC is very different. Different fighting games simply play differently. I would buy both...
SRK have the old posts about modding this stick on archive. Some links include:
http://forums.shoryuken.com/archive/index.php/t-84694.html
and
http://forums.shoryuken.com/archive/index.php/t-109890.html
A quick search for "tekken anniversary stick" on google should bring up the SRK archive...