I've had Mk1 Fathom 12s, mk1 fathom 15s, mk1 fathom mono magged (vmo, Phil stares), akion nitrons and now MK2 fathom 15s.
The mk1 fathoms were superb. But the brakes were known to be a weak point so you either have to settle with the fact that you will need to modify and fit regular brake blocks (easy to do) or get them magged.
I found the Phil stares mono mag was good. But I found as a relatively new multi user at the time that it could be easy to blow up If I got the mag setting a bit wrong. Seemed to be a fine line between controllable and fast.
Then when I compare the mag to the current MK2 mag setup, the MK2 setup is a very powerful mag. The dial with clicker also means it's easy to know how much mag is currently on. I use 0.4 or 0.45mm line on my 15s, it's an incredibly safe reel, will full mag I can cast and it can land without needing thumb control. Rediculous really.
I am a slightly better caster now than when I started with the mk1s, but the mk2s sure are safe and cast a decent way as well. Great fishing reel. I've no reason to change them now.
The akios reels mag was superb also, but the frames are known to flex under load. I also hate the garish Farbon dip treatment. I still fancied trying a pair though but quickly moved them on for MK2 fathoms.