advantages for me were:
1. you can switch jobs much more easily
2. your employer will pay money into CPF fund (reaching 16% of your basic salary after a couple of years)
3. you can buy HDB property if married to a Singaporean
disadvantages are:
1. you might have to do NS but unlikely if you are a 'skilled professional'
2. if you are tight for available cash each month having to make employee CPF contributions might be awkward
3. if you leave singapore and want to get your CPF 'out' you *may* have problems if you ever want to work in singapore again