I'm using Targetz since it came out and it's an invaluable addon.
I particularly like the fact that it allows me a clear gameview compacting much needed info in the tooltip.
I am however using
Brunilde Units on occasion, when I need a way
to track target's targets in a fast changing environment or I need a more persistent indication.
The fact that I can hide or show BUnits frame easily allows me to keep my gameview uncluttered most of the time.
I'm not suggesting it as a substitute addon (I'm using both) but as a complement.