If you're staying at the Sahara, as I was the last time I passed through Vegas, the Monorail is a godsend and deserves SIX stars.
For everyone else, call it the "Lesser-of-Two-Evils"-Rail. I think the Monorail disappoints people because they expect it to be like the NY subway, cheap and with plenty of stations just a few steps away from wherever you want to go - neither of which is practical in Vegas. But in certain situations, it's a nice option to have.