I've just recently got my self some flys aswell and have been super impressed with them. Just got back from a few weeks in WA so I've had lots of time to really test them out. I'm using the 5.4, 4.8 and 4.2 with specified mast for each sail. I'm not sure how the mag test came out with them being hard to rig? I've found as Dan has said you simply just rig them to the specs and the tweak to suit the conditions. Batten tension does however play a big factor, I was suprised how much more power I could get out of my 4.2 when I increased the batten tension.
The 4.8 seems to be a favourite for everyone whos used the flys, If i were to have one sail only it would definately be the 4.8. The low end power of the flys to me seems bit different to a sail like the NP Alpha, Gaastra Poison or Severne Blade. The powere seems to generate much more effectively from working the sail with a few pumps, whereas a power sail just powers on. The benefit of this seems to be a more neutral feel when manouvering with out the drawback of no power when needed.
As far as contruction goes my 4.2 copped a whoping on Gnaraloo reef last week and got a few gashes up the luff but to my suprise theres a double layer of material so the sail was still completely usable after a few big gashes as it only tore the outer layer. My 5.4 also copped a whopping at Windang before xmas with a solid 6ft wave tearing it down, the good news nothing broke (mast included

).
Hope this helps you make your decision.