I played a number of all-Guilds games last night and then started mixing in the other cards and came to the opposite conclusion. The Guilds-only boards were neat to learn cards, but were Soothsayer-BM and then Advisor, Masterpiece-BM (with some Butcher anti-stall assistance).
However, mixing Guilds into all the cards, I got to do some crazy-fun things. I lost a Minion split 3-7 I think and then won with Mystic and Plaza helping me to snag 6 of the Colonies before my opponent* emptied the Provinces. I ran a Herald-Goons deck with only Loan for trashing against an opponent who was overloading on Tunnels. It was almost as fun as the Heralds-Goons-Caravan-Advisor game.
Games with Stonemason and Candlestick Maker are built-in 3-pile red alerts.
The Guilds cards, to me, are not in-your-face powerful, but are more subtle and seem to have the ability to turn many dull boards into amazing.
*oppenents were bots, and seemed rather incompetent, but perhaps an interesting backdrop nonetheless.