The XP is really nice (it continues to scale as you level up), I think it might be slightly slower than doing the highest level dungeon available though. That said it's a good option if you like it a lot more than running dungeons, plus the queues are probably much better for a DPS. I know a few FC members that have gotten 50-60 on a few jobs this way without any trouble, though you might still want to do your daily leveling roulette since it's a pretty big boost. It's even better than pre-patch since the final floors are quite a bit smaller now so they take less time, which is really the biggest reason why it's not as efficient as regular dungeons.