DH and I have rented twice and are planning to do so again in September (though we will be renting at the GCH pool in Sept since we will be staying there). We have loved it both times. The first time DH and I went with my folks and we had several friends join us. There was plenty of room and we didn't have to worry about trying to get that many chairs together.
The second time, was this past June (Carsland opening weekend) and it was just DH, DS (4 months) and I. We had an amazing time and DH could have the US Open on the TV. Plus we were able to request a pack n play for DS to take a nap in, so that was awesome.
As for whether or not is was worth it, the first time, it definitely was since the pool was really crowded (since it was
Disneyland half marathon weekend) and we had a bunch of people. The 2nd time it was not crowded at the pool, so we could have had chaises easily, but having a safe place for DS to nap in the shade made it worth it for us (plus DH really wanted to be able to see the US Open without having to sit in the room.)
Be sure to call and let the cabana concierges know if you are celebrating anything and who your kids' favorite characters are. You never know, there might be a few surprises in the cabana.
Oh and be prepared for others jealousy. heh We had a lot of people who wanted to know why we got a cabana and they didn't. lol