I'm not saying that I'm an expert or anything but of the four games I played with it at GenCon:
Game One- Engine game with cheap components (Menagerie, Hamlet), I was reliably drawing my deck so it usually felt like the better move was to buy multiple components rather than summon one, or to buy $5 cards like Haunted Woods. I don't think I bought summon at all this game.
Game Two- Storyteller board with Miser and Ratcatcher for trashing, Market Square and Treasure Trove for Gold gaining. Again, whenever I had five buying more Stortellers and Treasure Troves seemed like the higher priority. I definitely didn't buy summon. I got second, but no one else used summon either.
Game Three- No +actions, I played BM/Gear. I did buy summon once, to summon a Warehouse at a moment when I knew my deck needed sifting, but it was a hard call between that and Duchy.
Game Four- Heavy engine board with Peasant, Colony, lots of other stuff. I summoned a Peasant on the second shuffle, I also put my -$2 token on Lost City and summoned that a few times. But again, a lot of the time it felt like a better move to spread out my buys between multiple components rather than summon one.
Summon seems like it's strong, but I'm not sure it's going to be an auto-buy on a lot of boards.