Steve, register and we'll give you more help than you can stand!  Otherwise, nothi...  /forums/images/graemlins/grin.gif ... just kidding.
The northern generators are nice units made in the Midwest.  You'll find the same generator head used on several manufacturer's models.  I have the 13/10 kW unit.  Think I saw it recently for $100 off their regular price.  Yes the tractor must run at near full speed, but that's what an engine mounted permanently to a generator is going to do anyway.  It's not a problem.  Also, the tractor govenor keeps the RPMs constant, assuming you have a decent tractor.  Slight variations don't matter anyway, as most US equipment can stand several Hz of variation and some as much as almost 20%.  Some electronics, computers etc, don't like too much variation, but unless you have a very poorly performing govener, you shouldn't have any problems.  The PTO generator is a great invention - no additional engine to start periodically or maintain.  There's lots of other info (and opinions) here at TBN, just start searching.