Will you remain loyal...?

Yes. Of the three chains where I spend 100+ nights a year, Hyatt is the most reliable for meeting my needs. (I'd spend more time with Hyatt if they had more hotels in the smaller cities where I often do business.)

Why?
StayFit - 24 Hour gyms with Life Fitness equipment. Double bonus.
Faster Free Nights - I don't count on them, but enjoy them when I get them. Typically, I earn more than I can use.
Property Maintenance and Utility - Sure, there's Monterey, CA, but many have undergone recent renovation...at least where I travel.
Diamond desk - Few complaints.

Edited to add:
Having read through some (not all!) of the previous posts, a few additional observations.
I did use some stay certificates in 2006 and found the experience unsatisfying. No one could tell me whether the nights would count, and whether the charging of incidentals would make a difference. T&Cs weren't clear.
I seldom stay in upper-end Hyatts (Park, Grand); my employer won't typically pay the higher rates and, during personal travel...well, I'm a poor man.

By the way, something of an exception to the 'rule' above, I'm booked for the Grand Hyatt New York from January 1 - April 13, 2007. (I dont' know my schedule beyond that date.) How's that for loyalty?