It's the GBA version, minus some bugs, with a significant aesthetic overhaul. It has the option for the DS music.

It has the Interlude, which is all new. It also has The After Years complete without the piecemeal Wiiware BS.

I'd say it's definitely worth it. I got it recently. It's 5th time I've bought the game and while I'd seen screenshots that showed how pretty it was, I was really amazed at quite how gorgeous it was once I started playing.

I'd say it's way more like the Tactics update than a ruined remake. It's got the best translation to date and some options like auto-battle are fantastic for speeding up grindy sections of the game. Oh, and did I mention how pretty it is?