It's a big university - the largest in the country after the OU, which is obviously a completely different kettle of fish. You can get lost in the crowd if you want to, but equally if you want support it's there for those that ask for it, and the mass of 40k students is rapidly broken down into people you know from halls, society and course. It also means that there's the critical mass of students necessary for all sorts of esoteric clubs and societies, and the funding for some top quality facilities and a lot of capital spending on the campus.
The cheaper halls are entirely functional, and an extra £10/20/whatever per week is invariably more valuable to a student than nicer fixtures and fittings; it all meets an acceptable minimum standard. No point getting an en suite - there are cleaners for the shared bathrooms, and when they all move into shared houses for second year they'll have to have shared bathrooms anyway so they may as well get used to it as freshers. Manchester is unusual in that it guarantees accommodation to all students regardless of whether they come as firm, insurance or clearing applicants; a rarity nowadays, and a great advantage.
Crime does happen in Manchester, but in general if you follow the usual rules you would in any big city then you'll probably be fine. When I say the usual rules, I mean don't walk across Platt Fields Park after dark by yourself while drunk and flashing your iPhone. And lock your front door. Anyone who has been mugged twice has either not been following the usual rules or has been incredibly unlucky.
I've very fond memories of my time there, and would move back if I could; I regret moving away after I graduated.