Lately I've been staying at TI (another MGM/Mirage property). Before that, I usually stayed at the Mirage. Over my 4 or 5 day trips, I accumulate 15,000+ points on my card ($150+ cash back).
Occasionally, I'll get offers in the mail for free 3-night packages, usually Sunday-Thursday. More often though, when I'm booking my trips, I'll call the Player's Club and make a reservation directly through them. They'll look up my history, and set me up accordingly. With the above mentioned play level, they typically give me 4 or 5 free Sunday-Thursday nights at TI.
Before my last trip, I had gotten a free 2-night offer from Bellagio, so I booked that. Then I called TI and they gave me 3 free nights there.
The key is to ASK. If you don't play enough, when you call the Player's Club they may give you a discounted rate instead of free rooms. And before you check out of the hotel, again go to the Player's Club and ASK what they can do with your bill, if anything. Sometimes they'll take off meals that have been charged to the room. Sometimes, if you originally were billed for nights, they'll remove charges too. ASK, ASK, ASK.
Hope that give you some idea of play level vs. comps.