First off, by all means, get the tux for formal nights. You'll look and feel great.
I've long since purchased my own formal wear, but have used the rental service in the past. Friends have used it recently. Its hard to beat their service. The tux is hanging in your cabin when you arrive. You just wear it, hang it back up and forget about it. Be sure to check the fit when you first arrive. Minor adjustments can be made on board. If your an unusual size, I would recommend going to a tailor at home to get measured.
For Holland America:
http://www.cruiselineformal.com/prod_hal_m.html
Tim