I like IS as a a sort of garuntee. Even 1/focal length dont get you a sharp pic sometime. IS ensure u almost surely gets it (note almost ). Its also great for low light long focal primes like 135 and 200. IS works best at longer focal length then shorter ones from my experience. Now a 135 F1.8 CZ from sony with a IS body alpha is a very gd combination for low light portraits hehe. BTW Im a canon user, IS not a nessecity but it sure is nice to have it.