I finished specializing my 109 F (33 battles with a 50% win rate - had some rough games the last few nights).
Overall, I've rather enjoyed the F - more than I thought I would. I did all right in it - even got Hero of the Skies in one battle (that we lost due to elimination). Most of the games (off the top of my head) I flew well even if we lost - seeing 10K personal points on average and having games where 3 or 4 sectures were capped and getting Grade III or better almost as much as Grade IV and V tells me I might be getting better at this game. Or, more likely, I had a lucky run LOL.
Speed, power and manueverability are all great. Yes, the guns are weak - noticeably weaker than the E. But I still managed to bring down 295 aerial targets with a kill ratio of 6.28. And a bucket of assists (which actually was OK as it finished period II for a couple of daily assist missions).
A quick glance at plane compare in the tech tree tells me the G should be an improvement over the F, at least gun wise. But I see it has a 30mm, and I've heard that's a hard weapon to hit reliably with.
I'll pick up the G this coming week - and after that, it looks like the German Meschermitt line will be my next tech tree Tier VIII.