Interesting thread, Ben. I myself have tried both REI MTS and ExOfficio. REI's is not bad, but the ExOfficios just feel wonderful. They are so silky and soft that I often wear them around town. I like them so much I went back the other day and bought another pair, while the REI languishes in my drawer. I believe the ExOfficios are a fraction of an ounce lighter, too.
Now...I briefly (no pun intended) pondered returning the REI briefs to REI, but you all can be glad that I did not.
Now that we're on the topic, how many bring an extra pair of briefs with them backpacking? I've been bringing an extra pair so I can take camp baths without standing around in the nude.