The benefit of the Westin Flex trust is that it owns multiple resorts where owners can reserve at twelve months. Owning an HRA allows twelve month reservations at HRA only; unless you always want to stay at HRA, you will be making all of your reservations eight months out. Owning a Kierland unit has the same problem, although the MF's are much lower.
My own concern about the Westin Flex trust is whether it owns enough Hawaii weeks, ski weeks, and winter desert weeks to meet owners' expectations (people don't buy Westin Flex for summer desert weeks, but the trust undoubtedly owns many of them).
Flexibility is the key here, and people do value that. Being able to reserve some of the most demanded resorts in the system at 12 months (and start any day of the week) has several advantages in specific situations. We use our small Flex package to add a few days to our regular WKORV-N week without having to wait for the 8-month mark to play the SO lotto; particularly at peak times. 7 days on Maui is always a bit too short, and 14 days is too long. Our school district spring break is 10-12 days long depending on the year, so adding 3-4 days works perfectly for us. Later when the kids are older (we have four) - I imagine we may want to use our HO to add a studio to run consecutive to our booking so everyone has a bit more space (or we can invite friends/family).
This year I grabbed a spring break week in March with our WKORV-N unit, and then added three days (which returning on a Tuesday instead of a Saturday or Sunday saved us $300 in airfare per person -$1800 total - more than our MF for the package). I've recently cancelled that due to uncertainty over reopening, and picked up an early August WKORV-N week, also adding four days with the Flex. Availability was good.
In general, I've been fairly pleased with availability. It isn't all summer desert weeks. Right now I can book a 4th of July week at Nanea or Princeville in a 2BR (July 2 or 3rd check-in). There is a studio available at WKORV. Keep in mind we're nearly two months into the booking window and I can't see Maui with my WKORV-N home week at this point. There is no 2BR SO availability for Nanea at that time and low availability listed for WKORV-N.
The one thing that I have noticed that is lacking is 2BR ski and summer weeks at WRMV (I do see the occassional 1BR), so I wouldn't buy into flex if that is something you must have. Winter weeks at WKV (Feb.-Mar.) go fast, but they are there at the 12 month mark. WMH and WDW seem to have availaility pretty much any-time during the priority booking period for Feb.-Mar.
I was skeptical of the WFlex system at first. But the more I have seen and used the WFlex program, the more I am appreciating it. It's not for everyone and I would continue to recommend a mandatory resale week if you want Maui for 7 days, but to augment your Vistana portfolio it's not a terrible thing.