It's usually a good program if you get an upgrade. However, 2 of my experiences with participating hotels was negative.

First, if you don't get an email and you can see on SPG that plenty of upgrades are available it sets a negative tone. In my case, I got an email saying there was no upgrade available followed within a few hours with an offer of a PAID upgrade. While I'm sure the hotel did not violate the SPG terms and conditions it certainly felt like they violated the spirit of the program and left a bad taste in my mouth for that property.

Second, at the Sheraton HK I was upgraded to a Towers Suite. Unfortunately, I specifically booked this hotel because I wanted a harbor view. I repeatedly asked to be DOWNGRADED to a non-suite room with a view and my request was denied. Long story but I never heard from management until after I checked out at which point it was obviously too late.

To me it just seems like there are more possible downsides than upsides. Maybe if they adjust the program or possibly pick more customer service oriented properties than this will work.