We LOVE the Big Island and strongly recommend visiting. The stretch from Kailua-Kona down to Captain Cook offers some amazing diving, beaches, coffee and chocolate plantations, and the Big Island's best (and most affordable) dining. Hilo-side is beautiful with waterfalls, orchids, and old-town Hilo itself. And Volcano is a two day destination all by itself with incredible landscapes and the magic of lava flowing on a dark night.
The problem with HGVC is that it is in Waikoloa which is too far from any of that and is really only good for the beach, golf, Waimea/Parker Ranch (if you are into horseback riding), and the charming Hawi. So much as I would like to be able to stay HGVC we usually get two nights in Hilo, a night in Volcano, and the rest of the week somewhere along the southern Kona coast. Obviously, your mileage may vary.