I'm not familiar with how "nice" in terms of the people who attend, but in terms of ambience, it's one of the nicest in town. Beautiful stained glass and gorgeous woodwork. Very reverent-feeling, if you will.
It's nice both in appearance, and in that it is full of nice people. The Masses are also done reverently, and each Mass has a different flavor (11am is Extraordinary Form, other Masses are ordinary form. Afternoon and Evening Masses have limited singing, 9am has more, etc). The Dominicans do great homilies too.
It is a Dominican priory, unique in Portland. It's lovely. The Mass is celebrated in a pre-Vatican II style, with the priest facing the altar, and taking Communion at the altar rail. I've only been to the daily Masses there, but I love it.
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