Callaway:
Strengths: FT series is made out of space-age materials, long-standing enigineering knowhow backed by a very talented staff. All-around good.
Cons: None, really.
Titleist:
Strengths: Unmatched in the better player's department. Workable, accurate, a ton of custom options, and undoubtably the best looking clubs on the market. Not to mention Scotty and the Pro V1/V1x
Cons: Expensive, not much for high handicaps.
Cleveland:
Strengths: Wedges, decent irons, and the Hi-bore series really does work. Also have a good lineup of balls and putters.
Cons: Limited options, Hibore keeps getting uglier.
Ping:
S: Anser and Eye2 are legendary, all of their clubs are solid. Had the original fitting system.
C: No forged clubs, and lime green and orange? Really?
TaylorMade:
S: Really is No. 1 driver in golf. TONS of options available, and since a new one comes out every year, you can get last years model at an affordable price. Fairway woods are just as good as Drivers. Irons and balls are both decent.
C: New stuff costs an arm and a leg, wedges aren't great, and putters are crap.
And Mizuno has pretty much been summed up already.