New Game+ is definitely worth it. I finally started truly getting on top of equipment and combat on my second playthrough, and the extra explorable dungeons and secrets are brilliant.
Remember encountering doors that you couldn't pass in your original game because they're "sealed with the rood inverse"? Well, thanks to the ending,
(SPOILER)Ashley's now wearing the Rood Inverse on his back so you can now get into those new areas and encounter some truly phenomenal super-bosses and labyrinthine passageways.
Plus, you'll have the chance to start finding and hoarding all the best equipment and magic, till you're strong enough that you're one-shotting even the toughest storyline bosses.