Raw conch and abalone have a bit of a crispness to them as you bite. Geoduck is always slightly chewy. The taste should be mild, slightly briny and sweet. The Japanese name is "mirugai," btw; it's very common in sushi bars, and because it's so common I don't think it's fair to call it a delicacy at all -- at least not on the west coast. At a large Vietnamese seafood market in Rosemead, price for live geoduck, alive and in the tank, customer chooses and "catches" his or her own (with a pair of tongs), was around $6/lb, as of last week. By comparison, farmed Alaskan Coho salmon was $3/lb for a whole fish.
Bitterness in shellfish is not a matter of how to "fix," "compliment," or "disguise." It's indicative that the animal is not fresh and should not be eaten. If the animal tastes or smells at all like iodine -- pass.
My favorite ways to eat geoduck are raw, as sashimi, nigiri sushi; semi-raw, as ceviche; or fried. I especially like fried geoduck as an oyster substitute in a Hangtown fry (not that there's anything wrong with oysters!). As sushi or sashimi the traditonal garnish is a bit of soy sauce only. Geoduck is way up my list of favorite sashimi, but only in a trusted bar. As sushi, the flavor is quite delicate and I'd classify it as more a texture than a taste.
I wouldn't buy it myself, unless I was sure it was alive at the time. That is, I would not buy on ice but only from a tank. That's easy for me to say because I've got the luxury. If "on ice" was the only way to go, I'd give it a whirl.
When doing a geoduck ceviche, it's incredibly good with papaya and a garnish of avocado at service. As a fried clam, it's quite good, makes an excellent clam po-boy. As part of a Hangtown fry, it's a great late supper -- especially with bourbon or brandy and soda.