Onion and garlic characters have a lot to do with when the hop was harvested off of the bine. I'm sure the way that you use it can limit the amount of these flavors you get if the compounds are already there, but a properly harvested Summit crop shouldn't give off these flavors to the degree that it has a reputation to.
That being said, I've used Summit in 2 beers...an APA and a DIPA.
The APA was Summit/Citra, with the Summit being used for bittering and a 15 minute addition, Citra for 10, 5, and dryhop. I renamed this beer "Onion APA"
The second time I used Summit, which was from the same 4oz pack I bought from Farmhouse (I believe it was 2010 crop), I used it along with Columbus and Simcoe. I used 2oz of it...1oz at 5 minutes and 1oz at flameout. This beer, along with 7oz of simcoe/columbus, had no discernable onion or garlic flavor, but rather a pleasant tangerine note that Summit can be known for.
Perhaps it was the mixture of the other hops that did this, or the alcohol content, or whatever...but it's worth noting that the Summit used for earlier additions gave me an onion flavor while using it for late additions did not.
This is a pretty limited test on my part but maybe with others chiming in we can iron out some better details.
