Law is quite clearly not a useless degree. If anything I would put it up there with the most useful degrees. Certainly, if you discount law as useless, you are saying that almost all arts/humanities are useless which is bigoted and arrogant.
I saw someone said Oxbridge is the only place worth getting a law degree, and I felt like I should acknowledge this misinformation for anyone considering a career in law.
This graph was created by chambers student, it can easily be found on google with a quick search:
http://www.chambersstudent.co.uk/media/1345/trainee-university-backgrounds-all-firms-2.png?width=678px&height=584pxAll Russell Groups and beyond are represented. The data was also collected from the top firms in London, including the 'magic circle'. The Bar is more educationally elitist, but even so going to Oxbridge is not a prerequisite. You can find similar data all over the internet. Admittedly, the university you go to is very important in law, as it is a competitive industry and the quality of teaching and students at different universities will of course vary.
I also saw someone state few lawyers end up earning 6 figures. Inside London, this is also utter rubbish, although it may be true for regional firms. Although anecdotal, my sibling is a Nottingham graduate (2.1). They became newly qualified last year and earn 6 figures. That's after 2 years of training, where they were earning ~50,000. So within 3 years of graduating, they have reached 6 figures. Few firms do pay NQs such money, but many are around ~80,000 including the Magic Circle.
However, I will say that you shouldn't get caught up in salaries. My sibling's friend ended up working for a small, niche firm in London which paid ~60,000 to NQs. However, they work far fewer hours and the atmosphere of the firm is far more relaxed and friendly. Money is not everything.