Not very (I maintain a nice iPhone app that front-ends the data on legislation.gov.uk): the data on legislation.gov.uk is useful for Joe Public, but when it comes to the actual practice of the law is woefully incomplete. This is purely down to a lack of interest and funding on the Government's part, as well as vested interests by people who publish law and charge for it.
There is no free source of up-to-date UK law: many of the statute on legislation.gov.uk is not guaranteed to be up to date, and if you're looking to practice this is obviously no good at all. This is in itself very sad. The people who run legislation.gov.uk do a fantastic job, and hopefully access will get much better in the future.
