I found 50NL to be very frustrating, but then again, so are all levels. I have found that everytime I moved up in levels that I would play my 'A' game and would have a very successful session. The following session, however, would be the opposite because I'm beaming with too much confidence. I play on PokerStars and have found a huge difference between 25NL and 50NL, but no difference between 50NL and 100NL.