First off, our guide Alex, was great. He was very informative, answered all of our questions, was very experienced, shared his personal stories hiking across the country and was very patient with us. He made the hike a lot more enjoyable than if we were to do it ourselves.
The hike was about 6 hours long. We got there at 8, had a safety briefing from Alex and left at 8:45. Keep in mind that the starting location mentioned on Tripadvisor is incorrect. If you enter this location in google maps, it takes you to a trail head that is a quarter of a mile before the actual trail head. The starting point is the trail head that does NOT have the 2 latrines. We also had to take into account the construction work happening on the roads in Yellowstone. Google maps does not do a good job of accounting for the time lost due to construction. It took us 30 min longer than what google maps suggested. You can beat all of construction hold up if you plan to get to the valley by 7:30. Anytime after that and you risk getting stuck in traffic. Wait times can be as long as 20 min.
When we arrived, Alex was already waiting for us. He offered us coffee, some ozzie bites and a couple of bottles of water. And when our hike ended he offered us Gatorade (much needed!) The tour provides hiking sticks (highly recommend using these), a backpack, and some lunch (apple, snack bar, some candy and a sandwich). They also provided us binoculars (a must have in Yellowstone).
Back to the hike. You will spend about 40% of the hike on trails that you could potentially go yourself, but Alex took us to certain spots from where there was a higher probability of spotting wildlife. We spotted Bison (of which there are plenty), multiple herds of pronghorn, Elk, blue mountain birds and several ground squirrels. We spent a lot of time trying to spot Wolves and bears, but unfortunately we weren't lucky enough. We also found what appeared to be perfectly preserved (and heavy) moose antlers on our way up. That was a nice little bonus!
Keep in mind that the weather in Lamar valley (and Yellowstone in general) is quite unpredictable. We were expecting rain during the later half of the day but ended up experiencing a brief halestorm 2 hours early. While this might annoy (and hurt) some people, we actually enjoyed the experience and it made the hike more memorable. And it also helped drop temperatures in the valley. It was chilly when we left for the hike but the temperature rose to the late 70s by the time it was 10:30 AM. We then stopped by the river for lunch, which was great. We sat under some trees by the river, hoped to spot a bear, while enjoying a cold pleasant breeze and a well deserved lunch.
While the tour is on the expensive side, I would still recommend it. I think you can expect to see Bison, pronghorn and ground squirrel. Other wildlife really depends on the day and your luck. I would also suggest doing this tour early on during your time at Yellowstone. We did the tour on Day 3 and so the novelty of seeing a Bison and Pronghorn up close was lost. We had seen quite a few by then.