Pitbull unit frames have a nice energy ticker and a built-in mana bar while in form.
I have the upper left corner of my UI set up like so, as a one-stop-shop for almost everything I need to know while DPSing:
[my unit frame] [target frame] [little Demon window] [target of target frame]
[ORA2 tanks] [Chronometer bars]
It works pretty well. Demon is right there to show me if FF and Mangle are up (we run with two ferals, so it's worth checking), and Chronometer tracks the status of my own Mangle and Rip. (Chronometer was always a little buggy and is getting worse, so I'd recommend finding something else.)
I used to have an Omen pull-out aggro gain bar right with my target of target, but it seems that that functionality was lost in the rewrite for 2.4. Now I have to look at another corner for threat info, unfortunately.