I've got the hyatt card and will be getting a SPG card soon as well.
My two free nights at hyatt are Paris park hyatt Vendome. It's nearly $800 USD a night, so I'd say that sign up bonus is pretty stellar if you use it right. I also enjoy the platinum upgrade (upgrades where available, free wifi domestically), and the free anniversary night, which at a lot of properties is worth $150-200 or more.
SPG points are just so valuable that I can't overlook that card.
I personally don't get the hilton system. I've looked at rooms that are $169 a night that run 60-70k points per night. I look at that award chart and an amazed that people redeem anything with them. I stay at Hiltons sometimes, but I have gold status (through Amex plat), so I'd be happier to pay out of pocket, especially on discount travel sites and then take advantage of the Gold benefits. I don't have enough experience with the other hotel chains to be able to speak about their cards, but I plan on keeping the hyatt and SPG.