I am also very interested in this, and am very new to this travel card game. I've had a Chase United card for almost 2 years, and will cancel next month before I get my second annual fee.
Who has experience using the Chase Ultimate Rewards system? They say you can book any seat, any airline, but it is hard to know if that is the case without actually trying it firsthand. Also, let's say I have 10000 points worth $150, and want to book a flight that costs $200. Can I book and pay the $50 difference? Or do I have to jump through some hoops by, say, buying additional points at some crazy rate? That is how United points are. If you don't have enough points for the flight, the points become useless.
Also something to note is that the points will disappear whenever you cancel your account (although I suppose you could do the 1:1 transfer to an airline before then??). So if you won't be able to spend the $1500 + $300 + $300 travel credits by the time you want to cancel, you would likely end up claiming the points for cash at a 1% rate (so $1000 vs $1500). Still not bad.
Any other quirks others have noticed?