The answer to this used to be that it was a good idea for younger and better off to opt out and a bad idea for the older and less well off - with lots of room for debate about precisely where to draw the line.
But most experts now think that it is best to stay a member of S2P, unless you have a very good reason to opt out.
The May 2006 Pensions White Paper says that the government intends to end the opt-out altogether. This won't come into force for some time, but is probably another reason not to opt-out in the meantime.
The Pension Service has a downloadable guide to all aspects of contracting out which also covers occupational pensions.
The FSA factsheet Contracting out of the State Second Pension is also very useful.
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