I first played Seven Wonders a few weeks ago and was really impressed with the mechanics, but found with 6 newish players it was pretty random. Still, I loved the mechanics as someone who loves drafting Magic, so I looked a bit more into it. I've played it online at jeuxsurunplateau.com a bunch in the last week and have found it rather intriguing (even if it is in French). After playing 20 games or so and reading up a bit I think with 3 players it's very nearly as deep as Dominion; in such a game, everything you do effects the whole board and you theoretically know where every single card of that age is after the first card is drafted (you just don't yet know which guilds you haven't seen). There are a ton of things to pay attention to, and each decision you make will probably have implications on the way the game will develop. In large games you really can't expect anything to come back, but in 3-player you can pretty well guess what you're going to get back around. The ability to foresee what things might happen as the game progresses is a very important skill and adds significantly to the depth of the game.
There are a few things I'm not a huge fan of though. First, you have to memorize costs that are sometimes a bit random if you want to get better; I'd much prefer if things were standardized in some way so that it wasn't just straight-up memorizing. Second, and this ties in with the first, you don't know what guilds you're going to see until you actually see them and their costs are all over the map. Worker's requires at least one of each raw material, Trader's one of each manufactured good. Two of them have 3 Wood, two of them have 3 Clay. They just don't lend themselves to memorization, and the incentive to memorize them is lower than for the other cards, so it's pretty random when it comes to being able to build the guilds that are in each game. Third, I think the game is slightly too short. I think I'd prefer one more play per age or perhaps 3 more plays Age 3. That said, having a very restricted number of plays is part of what makes the game hard to master, and currently 2 plays per pack each Age balances the 3 player game quite nicely (part of why I think 3 more Age 3 might work).
So yeah, after playing a bunch of 3-player, I'm not too keen on playing 6- or 7- player much; it almost feels like the difference between Monopoly you play as a kid vs. the real rules.
edit: Ok, I'm probably overstating the depth of the game after thinking more about it, as Dominion is seemingly bottomless while Seven Wonders most certainly isn't.