Did my first solo in 08 after several group trips.  I was instantly hooked, for many of the reasons cited above.  Did a trip with a partner after that, and it was fun, but not the same.

Never got bored, never felt lonely.  Filled my non-fishing time with camp chores, swimming (stayed very clean on the solo trip for some reason!) photography, plant/wildlife observation, climbing up to see the view, journal recording and plenty of quiet contemplation. Always had more things to do than time to do them, but never felt rushed, except on the one open water risk I took (a la wally's story).

I'm heading in from Ely in a few weeks myself, and another partner is making noises about going along.  I'll be delighted if he does, but equally delighted to have another opportunity for a solo trip.