Great experience! Our tour guide (Marlin) was fun and engaging. He made an extra effort so we could get a view of the lava from the Mauna Loa volcano in the distance. The sunrise standing above the clouds was spectacular.
The bike ride down was fun, though a little hairy with lots of curves and hand braking. They put a big emphasis on safety and riding at your own pace. Some people flew by us, but we went down pretty slow, which is not super easy since it's all downhill! But if you can ride a bike you will be fine. And if you do get cold feet once you are already up there you can always change your mind and ride back in the van.
The temperature changes quite a bit from when you start your ride down to the bottom where bike shop is located so layers are a must. There are jackets, backpacks and water resistant pants available in different sizes at the shop you can borrow. (Hint if you look in the lost and found bin you can usually find some gloves too which are helpful.) You can keep these on as you ride down or good option is to wear while you are on the peak, which can be for about 45 minutes and then leave in the van before you hop on the bike so you don't have to carry in backpack later.
For the first half of bike ride it is still pretty cool since you are up fairly high but very quickly it will warm up as you descend. There are a couple spots where you are briefly riding up a small hill, but otherwise it's all downhill riding. I used the pedals only briefly the entire time.
If you use the timelapse option on your phone to capture the sunrise it's helpful to bring a small brace or portable tripod to keep phone steady. I wish I had.
There are nice bathrooms at the peak but the ride up is about an hour so use the bathrooms at the shop in Haiku before you go if need be (back of shop in the hallway to the left).
As others have said it's smart to book this excursion on the first or second day you arrive in Maui to make it a little easier waking up so early.