It's not that II is a bad system (although this new "Short Stays" feature really doesn't seem worthwhile.) In general, II is a good company. The problem is, the cost in DVC points for an II trade is really quite high for what you get, in my opinion. DVC is a very high-end timeshare system, so almost any trade through II will be to a less expensive timeshare. Also, many of the nicest timeshares in II belong to systems that give preference to their own owners. So, even though DVC is a great system, you'll have trouble trading into the Marriott Maui because Marriott owners get first dibs on any deposits from there, trouble trading into Westin St. John because Starwood (Westin/Sheraton) owners get first dibs on any weeks from there, etc.
Also, I really wish you could search II online with DVC, if only just to see what II will offer me for my DVC points! I love going on II's website to see what's available; I got a couple of trades that way that I wouldn't have asked for because I had no idea they'd be available. But, currently you can't search II's inventory online with DVC points.
I've been very, very happy with II, but I've never given them any of my DVC points. I learned very early on that it was much more cost-effective to buy another timeshare just for trading. I bought an inexpensive week just for II trades; I bought a fixed week so I didn't have to learn another system involving points or reservations.
As for Hyatt, I've heard that it's a good system, but I don't know much about it.