assuming that the difficulty can be increased at most 0.5% per block(adjusted every block based on the previous block time).
The advantage with fast block time is
-the difficulty can be adjusted more quickly
-more confirmations can be done for a given time(makes <50% attack harder)
-A solo-miner or small pool will receive a more even income of new coins
the disadvantage with fast block time is that the effective hashrate decreases which makes it easier to do a >50% attack. The typical blocktime for altcoins seams to be 60 seconds, however their is some exceptions such as fastcoin(12 seconds) and geistgeild(experimental, 15 seconds), both these have bean running for a large period of time, wordcoin tried a target block time of 15 seconds but changed to 30 due to a lot of issues with orphans.
The blocktime can sometimes be a lot less than the target block time, especially during instamining