I have been before. For Dominion, the level of play varies; some people are strong and others not so much. I played in round 1 last night and was able to win. To get a sense of it: the kingdom had lighthouse, fishing village, woodcutter, militia, moneylender, council room, tactician, mine, and bazaar; i was the only player at my table to go for double-tactician and the only who made use of CR/militia. Two of the players failed to get enough lighthouses to stop the CR/militia combo.
As for the con itself, there are
lots of board games in addition to tabletop RPG (mainstream and indie) and LARP. It's not "focused" on board games - it's very diversified - but if board gaming is your thing, there will generally be ~10 games going at once at any given time (see schedule
http://www.dexposure.com/d15board.html ), including several tournaments with $100 prizes redeemable at the "dealer's room" (which is mainly board game vendors).
I'm there to play Dominion and indie RPGs. My wife is running some Nordic-style "art-larp" - she's one of like 2 GMs in this country who does so. Also, so long as I'm promoting my awesome wife, she wrote a narrative non-fiction book on LARP published by Chicago Review Press:
http://lizziestark.com/leaving-mundania/ (see blurb by Tracy Hickman)
If anyone is coming i'd love to meet up for some gaming. There is at least one complete set of Dominion in the board games room (including promos).