Amazing, and absolutely worth that last mile huff'n'puff. The road is gravel and rock to get in, but well-kept, just bumpy.
Took my kids (10 and 6), and two less-seasoned hikers. Hydration is necessary, but the flowers, lakes and view from the lookout is solid gold, even when cloudy. This was a wonderful day outing with the family, the trail was well-maintained and challenging, and there weren't a lot of other people.
NOTE: The distance conflicts at every site/guidebook I looked at. The map at Rainier Nat'l said that it was just a touch over 5 miles round trip.