Here some feedback from our Nationals - 50+3 is absolutely killing metagame diversity. We had 8 swiss rounds 50+3 and cut to only top 8 for about 180 masters at our Nationals. That means that topcutting is pretty much impossible without a lot of luck.
That's why i didn't really prepare something new and played a cool deck where frieds and me got some success with during this season - Flygon. It was actually good and I ended up 23th with a 5-2-1 score. All of the wins were 2-0 or 1-0 and the draw would also have been a win, and that's where the problem shows up - you can't win any more if you are one game behind with a slow deck, but you can have your win be turned into a draw if you lose second game and don't have enough time for a third.
Evolution decks aren't bad (even if Muscle Band hurt them a lot), I think most of you can remember that Empoleon, Hydreigon and lots of other decks were played during fall Regionals, but besides Emboar and Blastoise, they are unplayable in 50+3 swiss with a top cut to less than 5% of the players. That even applies to the fairy decks, which have a really fun gameplay but are too slow for competitive gameplay in 50+3. As far as I know, the Flygon deck two friends and me played was the only non-meta deck in the whole top 32 list. Back in 2006-2012, we got a least one trip-winning non-meta deck in either Masters or Seniors each year at German Nationals.
Top 4 was made of Genesect only, because its the only deck that can consistantly finish 3 games within 50 minutes while winning most of them.
Maybe things get better with Flashfire, but the problem still remains.
I'd rather have 1-3 additional swiss rounds and go back to b-o-1 gameplay. The main reason players (including me) were calling for b-o-3 before - T1 donks - has been pretty much eliminated by the new first turn rules (which I really appreciate).
To add something completely different - a friend who is father of two children in Seniors/Juniors has (for now) quit the game, because his kids didn't want to wait for about 3 hours every tournament, since Seniors und Juniors have far less players than Masters and therefor less rounds which now take even longer. He told me that the 50+3 is a real problem for all families with parents who have serious chances and want to play to the end.