I joined the two-day trip (not on the Gibbon Experience website). In a nutshell if you're looking for the possibility of a gibbon siting the two-day is not the excursion for you. However, if you want to spend the night in a treehouse and do some ziplines I felt two days was enough.
As a solo participant I managed to join the two-day trip by showing up the morning of. Once signed up be sure to get the suggested gloves to help protect your hands as you'll use your hands protected by dodgy bicycle tire sections to slow yourself down. Also bring enough water for the first day or both days if you don't trust the water they provide. I brought along about 2.5 litres and it lasted me about a day and a half.
The first day started with about a one hour drive to where we met the guide. After putting on our gear we spent the rest of the morning on a steady upward climb. If you're in reasonable shape you will have no problem but most on my trip struggled a bit on some sections. After the supplied lunch (bring your own food for lunch if you're a veggie) we started ziplining with the longest around 700 meters. We arrived at the treehouse at around 2PM where an afternoon snack was waiting (mainly fruit and some roasted nuts). We had two hours to do as we wished before heading back out for more ziplining (some of which we had already been on earlier in the day). When we arrived back around 6PM the beds had all been prepared and dinner was ready for us. We chose to delay dinner and it was all still hot at 730PM.
Day two the guide had us up to head out by 630AM to either do more ziplining or go on a nature walk. Since the ziplines are mostly lines you've already done on day one I recommend doing the hike. Having said that our group went ziplining and it was good fun. We arrived back for breakfast and about a two-hour break before heading out for more ziplines and to make the hike out. After lunch at someone's house there was a ride along a very rough 10KM road and then about 40 minutes on the highway back to Huay Xai. If you're prone to motion sickness take motion sickness pills when you arrive for lunch.
Aside from the lunch on day one there were lots of veggie options. For dinner there were three veggie dishes, one meat dish, and rice. For breakfast it was three dishes of veggies, a dish of eggs, baguettes, sticky rice, fruit, and some sweets. Lunch on day two we had fried rice.
The highlight for me was the treehouse. The first level contains the platform for zipping in and out along with a shower and toilet with amazing jungle views, the main level contains the eating area along with space for 3 beds and the loft has space for one more bed. Just listening to the sounds of the jungle at night made the trip for me.
If you've ziplined in a western country don't expect the same safety standards. It's pretty easy to glance the side of your head against the wire you're speeding along and I left with one small burn on my hand because my bicycle tire (used to slow me down) was pretty worn out.
All of that said it's totally worth the expense. If you're on the fence about whether to do it or not go for it!