Asus isn't slipping, the only thing is their Republic of Gamers boards didn't really live up to expectations and are very expensive and have had a few quality control issues so avoid them, but any of Asus' regular boards (like the Deluxe or Premium boards) are pure quality and should be regarded as among the best motherboards available.
For s939 I'd pick an Asus A8N though a DFI nF4 will yield better overall overclocks
For AM2 I'd pick Asus hands down, consider Abit too though they're really making a come back and getting rave reviews.
EDIT: Wikipedia could be a good source
basically a chipset is either a North Bridge (on single chip solutions) or a North Bridge and South Bridge (on dual chip solutions, by far the most common these days) the North Bridge is basically the point of communication between the CPU, RAM and video cards (PCI Express or AGP) (though in some SLI x16 or Crossfire x16 mobos, the second card communicates with the South Bridge) while the South Bridge is the point of communication between all your IDE/SATA devices and your PCI slots.