I only have experience with Marriott. I find it is very property specific. Lately, I travel to Philadelphia, PA a lot for business. I am 2 for 2 on unsolicited suite upgrades at the Courtyard Marriott Downtown Philadelphia. I am 1 for 2 for unsolicited suite upgrades at the Marriott Downtown Philadelphia (to the presidential suite no less on a one night stay). The fact that most of my business stays are short (1 to 2 nights) probably helps my upgrade percentage. That said, previously I was traveling all over Ohio for business and my suite upgrade percentage was very poor. Occasional upgrades to a junior suite at Fairfield Inn's (thought frequently I just booked the junior suite because it was only a $5 to $10 up charge). Also did fairly well at The Renaissance Columbus, OH with 3 for 4 on suite upgrades during the annual Ohio Educational Technology Show. Upgrades to the CL floor were fairly common at full service properties. Most of the time on business, a suite upgrade is a very nice to have but not critical. For leisure travel with my family, it would be great, but that almost never happens. YMMV...

--Jon