Who here has experience playing with 5 or 6 players? A lot of people always quip, well just play two games, but personally I think that's actually worse (they're likely to not end at the same time, you split the group up etc.)
I only have base (basic cards wise), but I've found that 5-6 player both work well as long as you don't put cursing cards out (and I usually avoid attacks that stack in some way, e.g. Ghost Ship because it's horrid getting hit turn after turn, or Ambassador because nobody likes everyone's deck rapidly filling with junk). I usually play with a house rule that there are 12 supply piles, which generally helps reduce the dependencies on one pile.
The main thing I feel about this though, which is actually why I started the topic, is that in my experience, 6 player is more similar to 4 player (and also to 5 player) than 4 player is to 3 player, or 3 player is to 2 player. With 3 players, a lot of the strategy you'd use with 2 changes (e.g. PPR, key piles are more likely to run out, 3 piling is more common). With 4, things change drastically again (now the key piles are even smaller, and there are only 3 Provinces per player, making BM much stronger). But with 5-6, not an awful lot else changes (piles are even smaller again, and there's less Duchies to grab). It really makes me wonder, why are people so happy (or at least neutral) about 4 player, but despise 5+ so much? I think Dominion's sweet spot is 2-3, both of which are great, and with 4+, well I'm happy to play but usually I have a better alternative. And yes I do prefer 4 to 5 to 6, but it's only a small preference.