I've bought my last 3 iPhones off contract although the previous 2 (3G & 3GS) weere still locked to O2 (UK).
Bought my 32GB 4 this morning and although I'll still be using it on O2 when in the UK it now gives me the flexibility to use it abroad on a local network rather than pay roaming costs.
Advantage of "off contract", I just sell the last model in 12 months time and pick up the latest, no upgrade hassles!