dragon age 2 isabella should I use?

Answer 1

Isabella is the only companion that can effectively go down a dual wield dagger assassin build. If assassin damage is your thing and Hawke is not one, then definitely use Isabella.

Answer 2

No.

She's not a very good character considering the options you have in the game already and there's also a bug where if you reach max friendship with her (or Sebastian) it doesn't give you the intended bonus it should, it actually lowers your damage resistance and attack speed.

More info here: http://social.bioware.com/forum/1/topic/305/index/6575152/1