I've only played three times so, but the first two games were with 4 players and we had a blast. The trading was quick and rather cut-throat and the winner was definitely the person who traded the best and managed their fields the best.
The other night, we played a 7 player game which managed, somehow, to drag on too long in the trades, but be over too quickly going through the discard decks twice. Granted, it was the first time three of the players had played the game, so there wasn't much interest in strategy and way too much "I'll trade you for whatever you want because I'm being nice."
The person who won was the new player who seemed to have the least grasp of the game, leading some players to think the game was, overall, too random.
Allowing for the above excuses, has anyone else had an experience that leads them to think that 4-6 player games are better that 7? I haven't played a three player game, but I've heard the trading is more limited and more likely to be "gang up on the weak guy".