Yes it is accurate, but there are some caveats that require the results be ground-truthed. There are limits to the resolution of the data, so there is some averaging and extrapolation at this level on both the ground surface and tree tops. The LIDAR likely will not resolve the finest branches that make up the extreme top of the tree. It is measuring the distance between the tree top and the ground directly below the top, which might not be the same as the base of the tree. This is a particularly important variable on areas of steeply sloping ground. It works best on areas that are generally flat. But with all that being said, from what people have posted it appears to be a fantastic tool for scouting out an area looking for tall trees. You would still need to visit the site and determine the species of the tall hits and confirm their heights. For species that are not the tallest, the area still needs to be scouted and measurements made. If all the tall trees on a site are sycamores, the site still should be visited to measure other species and to look at species diversity, girth etc. So LIDAR a big thumbs up as a scouting tool, but sites still need to be visited, and if you are looking for old growth - the LIDAR tells you nothing about the age structure of a site. It could be a site dominated by young tall trees, while an older forest on a less ideal site may still be relatively short.
"I love science and it pains me to think that so many are terrified of the subject or feel that choosing science means you cannot also choose compassion, or the arts, or be awe by nature. Science is not meant to cure us of mystery, but to reinvent and revigorate it." by Robert M. Sapolsky