Hello there Thanks for this - we've been asked this before.
We would say as we always have that you should always bear in mind this is a public forum, searchable by Google, legally quotable by all and linkable to by all and sundry.
Clearly having an open forum brings with it risks but it also brings with it great benefits we've always felt. Openness means volume of users and volume of users means Mumsnet in its many guises is available to anyone who needs advice 24-7. It also means fresh faces, differing points of view and debate, and the wisdom that comes from a very big crowd - wrong or dangerous advice doesn't tend to last very long on MN.
Whether the risks outweigh the rewards for each individual only they can decide. Clearly there are basic things you can and should do to protect yourself (ie not reveal basic contact info, namechange to reveal personal stuff etc etc). And bear in mind we are always happy to delete injudicious posts - just report them if you're worried about having revealed too much. HTH MN Towers