Last May I rented from Go Cycling Maui. I had the kids and wife with me who weren't interested in getting up early to drive to the other side of the island to try out one of their daily ride packages. I was crossing my fingers to get a bike with decent brakes. They rented me one of their Litespeeds for only $5-$10 more and told me the best way to get to the Hana hwy from their shop.
All I have to say is 10/10. The bike was unbelievable for a rental. Perfectly clean, tuned and in great shape. I went through 3 separate rain storms and lots of wet road riding and felt really bad bringing it back dirty, but they didn't mind a bit. One of the shop guys gave even me a nice discount on chamois butter since I didn't have the bike out for the full day.
The road I chose to ride was the Hana hwy out to Hana mostly one lane road. Despite the warnings I got from just about everyone who had been to Maui, it was heaven on a bike. I paced behind a minivan for 45 minutes until they finally turned out to let me pass. I didn't have many people on my tail at all. You can't see half the scenery from a car, let alone stop whenever you feel like it. I highly recommend this road, nice mix of descents and climbs, very nice pavement, beautiful curves, fresh, clean air.
The wind wasn't bad at all. Warm rain is no problem. I just had a short sleeve cycling jersey, my bib shorts, helmet, glasses, glasses, shoes and pedals. If you stick on this side of the island, you have a lush canopy hanging over you. Other parts of the island don't have as much vegetation, especially if you climb Haleakala.
Do figure out a better plan for lunch than I did. All I ate was banana bread from a stand and some clif shots. By the time I made it to Hana (2pm) most places that sold food had put away their lunches. Either start early enough, or bring something that can get you back to the shop.
I'm envious of anybody who is planning on riding in Maui. It's definitely an item on the checklist of things to do before you die.