We've been going to Maui every year since 1989. We love it there, got married on the beach at Kapalua Bay, and not surprisingly, finally bought a timeshare at the Westin Ka'anapali in 2003. We should have bought one sooner, but had never really learned much about timeshares before!
How many will be going? Do you have kids? Whale watching season is a nice time to go (usually late November-early April, or somewhere in there). Summer is popular, and the humidity is higher. Before we had kids, we would go in May and October. We've also been there in February (a bit more rain) and September, November & December. You can't really go wrong with any month

.
Snorkeling is great, beaches are great, driving to Hana is beautiful, taking a bike ride down Haleakala at sunrise is invigorating...eating out is fun, luaus are entertaining...there is a nice aquarium, some nice shopping..
We use the pools and go to the beaches, watch the sunsets, BBQ and relax. The kids are happiest playing in the surf and sand.
We've stayed in privately-owned condos, hotels and timeshares. If I can help, let me know!
Oh, and if exchanging through II is too uncertain for you, I'd recommend renting out the
DVC points and using the proceeds to rent the exact week and resort that you want, or saving your points and just using your cash. To me, that's easier than waiting around for an exchange to match.