Already a topic about this, but I'll answer anyway...
Right now out of 24 reviews it's averaging a 78.7 on gamerankings.com. That puts which is actually higher than the new GH Aerosmith but significantly lower than GH 3 which scored an 86.7 on Wii.
I don't know if it fits into the old DS, I'd say yes but you'd be best to ask at a game store.
I haven't played the game myself so I can't form an opinion but reviews have been either hit or miss and of people I've talked to who are GH fans some like the game, others don't.