messengers here in montreal use em daily. i've seen some on the road for 10+ years.
the plastic buckle will not fail. IMHO some of the bags out there are a tad overbuilt.
the nicest thing about them is they cinch down real flat when they're empty. plus they loosen very easily one handed, as in one-handed pedalling. i don't like bags which seem like you need a secret 2 handed handshake to swing in front. makes it hard to hold a slice and pedal.