I wouldn't recommend someone travel across country to do the Hennepin Canal Trail, like they might do for the Katy, GAP/C&O Canal, or Erie Canal, But for a more modest tour, I highly recommend it, especially if you can combine it with a train ride. The surface is not as good as newly laid asphalt; but is at least as good as the limestone/pea gravel found on all the long distance trails.
In general, I find rail trails more interesting than the Hennepin. There is something about cruising into the center of town on an old rail line bypassing the road-town sprawl that inflicts most towns. As others have pointed out, the Hennepin does not go through towns. Its purpose was to connect the Illinois and Mississippi Rivers; and it was built long after the towns had been settled. While it's politically correct to run an interstate highway through the center of town, I doubt the towns would have appreciated being dug up for the canal. In any event, the services in the towns are only a mile or two off the canal
I second the comment posted in "The Chainlink" that the place needs more use. In the three days, I saw maybe 3 bicyclists and the same number of walkers. About the only people making use of the canal were fishermen.
Finally, I was able to obtain a good map and mileage chart by writing to the state park visitor center.