This is probably the most upscale vegan restaurant in Portland and it's definitely a good place. For one thing, it's casual, cozy and unpretentious; for another, it's not terribly expensive (though it's not cheap either).
We started off with the mushroom pate appetizer which was well-received by everyone in the group. Basically it consisted of mushrooms and herbs mashed together and served with crusty bread. The two non-vegetarians in the group really raved about this dish and considered it as good as anything meat-based that they've had.
For my main course I had the arugula walnut pesto mushroom ravioli ($17 for a large portion). This was a great, savory dish that looked and felt like spring: it was homemade pasta covered with leafy vegetables of differing shades of green. A wonderful dish. The portion size wasn't huge (don't expect leftovers) but I definitely felt the price was justified given the quality of the food.
For dessert I had the rhubarb tart with cashew-thyme ice cream. This was okay, at best. I felt the cashew-thyme ice cream had a weird texture (the best cashew ice cream I've had is at Lula's Apothecary in NYC), and the rhubarb tart tasted pedestrian on a whole. I tried my friend's chocolate mousse tart and it too was middling.
The service was friendly. Expect a wait on busy nights. Keep in mind that reservations need to be placed 24 hours in advance.
I also tried some of my friends' dishes. The other main pasta dish, which came with mixed vegetables, wasn't bad, but wasn't great either. The minestrone soup was tasty, but seemed too basic to be on a menu at an exclusively vegan restaurant.
In general, if Portobello had somewhat more sophisticated dishes (dishes that you are unlikely to find on menus at non-vegan Italian restaurants), it would warrant an extra star.
And, while I understand that Portobello is trying to move away from the world of mockmeats, I think there's room for things like homemade seitan or tofu, if not necessarily Gardein products and Daiya vegan cheese. The other vegan Italian joint in the US, Chrissie Hynde's "VegiTerranean" in Akron, Ohio, has similar prices but offers a lot more food for your money. They don't focus on local, seasonal ingredients, but they do a hell of a job showing people that vegan food can indeed be tasty. Portobello and VegiTerranean represent two different models; I like both, but I think VegiTerranean won me over more.