We stayed at the Wailea in February for 5 nights on points. We chose this property because the weather is supposedly better here than Lahaina during the winter.
Pros
We loved the area, very calm, peaceful, upscale and beautiful.
Easy walk to a quiet beach
The view! Great views from our room, the infinity pool, the 'beach' and the restaurant.
Quiet resort. There were few children around and the hotel wasn't packed.
Close to shops and restaurants, there's a mall just steps from the Marriott plus the hotel has a Starbucks, a few little shops and everyday artists set up stalls & sold their wares near the lobby.

The cons
Not on a sandy beach. The shoreline in front of the hotel is black lava rock. (Less than 5 mins walk to an actual beach.)
The pools were too cold for us! We did swim once or twice but in February the water was just too cold to stay in for long.

Overall we loved the property and would return. We knew the 'cons' before our stay and so we weren't disappointed and planned to leave the property frequently (whale watch, snorkel tour, trip to Hana, drive up Haleakala) and spend our time at the hotel relaxing with drinks at sunset.
We also stayed at the Hyatt Regency for a couple of nights and spent most of our time there swimming in their fantastic heated pool and the rest on the beach!

If you want to spend all your time at the pool (swimming) and the beach then this is probably not the best choice for you