I'm a typo addict, 90% of my portfolio are typos , I've registered/tested several typos and from my experience, the www-, c.com and l.com typos are the worsts.
But there are some exceptions of course.Per example, I've one "c.com" typo which real site have an ovt score of high xx,xxx and the typo gets about 8 visits/day.I also have a l.com with xx,xxx that gets almost the same visits.
I've registered several www- typos, most of them with high xx,xxx OVT score and none of them worth to keep.
I'm sure that there are some www- typos that worth keeping, lets say per example an high paying keyword site that gets about 2 or 3 visits per day and with a ctr of 90%...
IMO the best to reg are the missed word typos.Some www are ok too.
Overture could be a good indicator but shouldn't be the only reference to check before buying a typo.Google can help a lot!
To finish, I've a "wwww" typo (yes, its four W's ) that gets about 7 to 9 visits per day...
If you're freak like me, than you should think and re-think several times before you reg a typo.
Some could be very profitable but in 60% of the cases you will only get 4 or 5 hits per day...If you have a high paying keyword site with good ctr then you hit the jackpot!

Thats my 2 cents