Dan,
I don't know if there are really any cons to the FZ-12 especially if you know when to put it on and when to take coils that size off. As trash increases it's not the coil of choice. I am sure you're well aware of those issues... it's mainly being mentioned for other readers.
The FZ-12 has done a good job for me just about every time I have put it on. It's has found deep silver that I have walked over with other coils and machines. I have also pulled iron out of the same hole as a coin that my other 3D coils passed up. I also found a SLQ pretty deep. One thing I have found out is that the FZ-12 on my 3D loves very deep nickels. I have found alot of deep nickels with the combo. I have never seen a combo hit nickels so deep in my area. I had a small area that I detected and found quite a few mercs in. After I pounded it hard I (in a passing thought) figure I'd tie on my FZ-12 which was new at the time. Walk over and basically had two Buffalo nickels in under 5 minutes. That scene has happened alot since then. As Dave said it's about the weight of the 10.5 coil. But, it is pretty sizable. My opinion of the coil is very favorable. It's typical excellent SunRay quality.
-Bill