Shell's Kauai Coast Resort
Now I will admit that my take on this is probably a little bit skewed as I am sitting in my gorgeous ocean-front unit while I write this, but Shell's Kauai Coast Resort is easily comparable to the Hiltons, Marriotts and Westins of North America.
I have never stayed at a Hilton, Marriott or Westin TIMESHARE, but I am/have been a Marriott Platinum, Starwood Platinum, and Hilton Gold VIP member so I am very fond of these brands and stayed more hotel nights than I care to mention. Hands down, the three Shell VC resorts I have stayed in (Kauai Coast Resort, Kona Coast Resort, and Donatello in San Francisco) are easily of the same caliber in terms of quality of amenities, service level, etc. And the real kicker -- Shell points are the least expensive on the resale market (they don't have ROFR, which is a real benefit when you're buying, not so much when you're selling though). It was one of the factors that got us looking at Shell points, and eventually of course a purchase.
Another TUG member (mepiccolo, I think) has stayed at more than one of these timeshare brands in Kauai, and if I recall correctly, gave the edge to Shell's Kauai Coast resort. Do a search in the Hawaii threads using her username.
Anyway, you obviously should make your own decision on this, but be sure to consider the Shell system. One of the disadvantages is that they are not as big and world-wide as the other three.