I've not lived in or around Manchester for over 30 years, but I visit the City centre, and go out there, more now than I ever have, since my son moved up last year. I think its far better than it was in the early 80's when I often had to get buses in and out, in the evenings (going to and from work at the airport), when it seemed quite an intimidating place full of scrotes (as did the buses).
I'm certainly not there every weekend by any means, but my lad (26) is, and he never really tells me of any trouble, in fact the only mither he's had in a year there, was a with a young City player, so I don't know if its just the places people visit. Yes in and around Piccadilly Gardens there are dealers, and there is a lot of "homeless", but I think if you are sensible there is no need to interact with them.
Despite the fact there are a lot fewer police visible than back in the early 80's (apart from football days funnily enough - who says football fans are treated differently ?), there's far more people living there also, so I'd say there is less trouble, OK much more CCTV helps with the fewer police.
I currently live in a smallish market town in the SE of England, and every day we have the same stories about relatively minor crime in the local media, much of it is racial in nature. I would bet the proportion of stories to the size of the town, its far worse here than in Manchester, in fact I rarely use the town centre here at night, because its full of drunk/drugged scrotes looking for trouble, but also I know where to go and where not to go, so its all easily avoided.