When I lived in North Carolina, this was the meat substitute I encountered almost exclusively. In a culture that loves simple food with an obvious and recognizable farm origin and that tends to avoid anything more exotic than Tennessee barbecue, tofu simply wasn't an option.
I never found a really good portabello there: most restaurants simply slipped the giant mushroom in place of a hamburger, and although both may be juicy and savory, a portabello is too watery, too slippery, and too simple to be an effective substitute for the latter. Ground beef has a range of complementary flavor notes, but portabellos only have one, the lone and overpowering taste of mushrooms.
I found the first really well prepared portabello at the D.C. locavore restaurant Founding Farmers. (Fellow Atlantic Food Channel writer Zeke Emanuel reviewed them somewhat unfavorably--I must respectfully dissent and say my experience was very good.)
By cooking it in soy sauce, they managed to drain much of the mushroom's oceans of water while retaining its juicy texture, and the soy nicely cut the heavy mushroom flavor to more palatable levels. I was unable to secure their recipe, and am not up to the task of formulating my own, but Eleanor Barkhorn had no trouble and generously shares her recipe here.
Founding Farmers serves their portabello with mushroom risotto (my own recipe for which is here). Eleanor serves it with a lentil salad. I recommend treating it as the rustic American farm food it wants to be and serving it with sweet potatoes, either mashed or as fries, collared greens, and watermelon. View recipe here.
Max Fisher shows how he's
Making the Most of Going Meatless
with recipes and techniques for spicing up vegetarian classics.