Had a 520 for a week, just returned it as it kept freezing.
While i was using it though the accuracy was always around 25-30 feet, whereas a garmin 1000 i was using on the same ride to compare with gave a constant accuracy of 9-10 feet. Glonass enable on both. My old Twonav Ultra gps also gives an accuracy of 9-10 feet nearly all the time in this area.
Just wondered if i had a bad unit, or is the accuracy on the 520 not as good as some other units. Might try another one if mine was just a faulty unit.