I have seriously looked at Algonquin as a poor Quetico substitute & know a number of people who have paddled there, but I have yet to go.  The 13 vs 20 hour drive from where I am is enough to make me think about it - but mostly I think I might be able to get my wife to Algonquin before I could talk her into Quetico.

It's proximity to Toronto & major US cities make it pretty crowed compared to Quetico, but I understand if you drive to the north/northwest entry points, you can get away from a lot of the crowds (had friends do just that in '04 & enjoy it).  I've heard good things about the park from those who've been there - scenery is still very shield-rock/Q-like from pics I've seen.

Typical portages seem long by Quetico standards; portages are marked & I believe campsites may be too.  The road, lodges, campgrounds & other facilities in the south just attract lots of people.  It's not 'wilderness' there.

I'd also be interested to hear 1st hand reports of those who'd paddled there & tried to get away from the edges/people/facilities.
-Tom