Did a fantastic job at Blackburn. With a very average squad he made them regular qualifiers to Europe. Little to no money to spend, he did wonders with what he had. Similar to what Moyes has done with Everton I'd say (although not quite to the same heights as Moyes).
Yeah, I'll admit he did crap at City, he wasn't a good fit to manage City. A bad appointment.
Did a decent job at Fulham IMO. Nothing to set the world alight, but didn't exactly flop there.
As for QPR, he needs to leave there ASAP, or he risks becoming a forgotten man.
As bad as he's been at QPR and City (and Fulham if you insist on counting that as a bad tenure), the job he did at Blackburn and particularly Wales have been nothing short of brilliant.