Nature of a forum is people either complaining, or looking for help - very rarely do you see a post of everything's FINE.
Some issues are caused by user error, some phones are defective & in some cases it's a combination of apps, which together (or one app) don't function properly.
I've got an AT&T Lumia 1520.2 on Lumia Cyan / WinPhone 8.1 ... with Dev Preview Update 1 & my phone is running fine - the only thing I see (as a result of DP Update 1) is increased battery consumption by IE. There are myriad posts on here about how good the Lumia 1520 is ... check here (post #40) >>
http://forums.windowscentral.com/nokia-lumia-1520/300837-2.htm#post2728768
BTW - the CYAN upgrade brings WinPhone 8.1 with it, if leery, hold off on the Dev Preview updates.