I've actually had quite pleasant stays there. I've always found them to have a modern lobby and clean, comfortable, well-kept rooms. My only irritation has been that they've started to charge for parking in an area where that really shouldn't be necessary.

Residence Inn Chandler/PHX is very nice and the staff are pleasant.
Marriott Fallsview Niagara Falls is a family favorite

Fairfield Tempe was ok

Recommend:

FI Verona NY. Clean crisp sheets. Nicer than most FI's
Charlotte Marriott City Center, NC. Very nice lounge.
Wentworth By The Sea, NH. Very attentive staff and nice lounge.

New one (for me) that I can comfortably recommend....

CY Norman, OK. Stayed last month. Friendly staff, clean, comfortable. Several good restaurants within walking distance and scads more within a five-minute drive.

Great:
JW Hongkong
Rennaissance Shanghai Yuyuan
Marriott Gaithersburg, MD
Residence Inn Houston Northwest

Not so great:
Courtyard Portland Airport - depressing location...

Agreed on this hotel. Stayed one night a couple weeks ago and they went above and beyond at every step. I had some boxes delivered to the hotel and, without asking, the boxes were put in my conference room the next day. (When I checked in, they told me the boxes were there. I said I didn't need them until my session the next day. That was all they needed to know.) I asked if there was an ATM, was told it was in the connected building, and then was walked there. A late check-out was no issue.

Gaithersburg has a nice walking/jogging path. Of course it's a top notch property, given its location likely means many Marriott bigwigs stay there often.

The CY Portland Airport isn't in a great location, but I've found it to be an otherwise good hotel. It's convenient if you are getting in late or leaving early in the morning.

I've stayed in many Marriotts, but there are only a couple that stick in my mind as good/bad:

My favorites (one's I will see reasons to stay in again):
-Chicago CY Miracle Mile (they always seem to remember my name, even when I haven't been there for a couple of weeks !)
-Marriott City Center Minneapolis - those corner rooms are outstanding !
-Fairfield Omaha (I expected the worst and was pleasantly surprised !)

Avoid if possible:
-Marriott, Dayton Ohio. Can't even figure out why Marriott allows this property to be called a Marriott !
-JW Marriott Cherry Creek (Denver, CO) - uncaring staff, mechanically poor property and, even for breakfast, the restaurant was the worst I've experience in any Marriott. And this is a JW Marriott???

Sleep in my rental car before I stay in again:
-Marriott San Antonio Riverwalk - totally uncaring staff, totally messed up front desk operation. I could go on and on about the problems with this property, but I won't.

Let me add a few.

Definitely Return:
Residence Inn Toledo Maumee.
Grand Bohemian Hotel Asheville.
Marriott Richmond Downtown.

Terrible, terrible, terrible.
Residence Inn Baltimore Downtown.

I love this category! :D

We were very pleasantly surprised with our five night stay at the Courtyard Shanghai Xujiahui. This hotel beats most regular US full service Marriotts by a considerable distance, especially if you have elite status. The CL there is fantastic with great breakfast and evening selections. Staff there are unsurpassed in their ability to deliver. Views of the city from most rooms are jaw dropping. And, best of all, the prices are unusually reasonable considering all of the amenities one receives. I'd like to go back to Shanghai just to stay at this property, although I understand the other CYs in Shanghai are of a similar quality.

GO:
Grand Flora Rome
Bangkok Resort and Spa
JW Phuket
Marriott Suites Dulles
JW Orlando
Baltimore Marriott Waterfront


Dont Go:
Marriott Full Service Dulles (Didnt like property layout service a bit lacking)

both lists not comprehensive :D


(Many other properties are exactly what we expect these are most of what stand out.)

Best and Worst
 

Full service Hotels only:

Best: Renaissance Chancery Court London
Bangkok Marriott Resort and Spa
Beijing Marriott City Wall
Athens Ledra Marriott
Sydney Harbor View Marriott


Worst: Washington Dulles Marriott
Norfolk Waterside Marriott
Karlsruhe Germany Renaissance

Fairfield Inn:

Worst almost all of them

Springhill Suites

Best almost all of them

Residence Inn Santa Fe on Galisteo was fantastic - huge room, great service, great breakfast and amenities, all for $75/night. I'd live there full time if I could!

Recently stayed at the old FS in downtown SLC. The bathrooms were very small. Basically only one person at a time could fit. On the other hand, front desk staff was exceptionally nice to my kids. Also, concierege was very friendly.

Stayed earlier this year, and going back next year, to Ko Olina timeshare property. We found the employees to be exceptionally nice across the board, rooms nice, food good, etc.... It was a very pleasant stay. I highly recommend it. Much nicer than the Maui timeshare in my opinion.

Have to agree that Ko Olina property on Oahu was the best of its kind we've encountered. We spent a great week there last December and hope to return soon.