Using the fitness center at an other resort?

Question for you all : We're staying at the Beach Club, but really like the Muscles & Bustles Health Club at the Boardwalk.
Do they let disney resort guests use the fitness facilities at other disney resorts or are we stuck with our own gym?

I don't know the policy but I don't recall them checking any room keys when I was there a couple years ago so if you happen to wander in, I doubt anyone will bother you. You are paying for a deluxe hotel. I don't think it would matter. JMO

You have to sign in and show your room key when the attendant is on duty. My sister used the gym at BWI last week, she also had to note her room number on the sign in sheet.

I'm not sure what the policy is for non-resort guests (that are staying at a neighboring resort), though.

Yes, WDW resort guests can use other resort fitness centers. On many of my trips when I stayed at a moderate, I have used the gyms at the deluxe resorts.