I don't use it myself but have heard good things.

The most important point is GET THE COMPANY MATCH!! ITS FREE MONEY!!

The union includes a bit about 401k enrollment probably once a quarter in its emails, and it's always stunning to me how many people don't partake in the program. I get that some might need all the money they could get, but enrollment is maybe 2/3 of the pilot group and that's it.