I'll go against the grain here and state that Magic Circle training can be extremely hit and miss. I trained at a MC firm and really didn't feel like I learned much at all. The size of the transactions meant that you worked on a tiny portion of the deal (normally compiling contact details and lists), you had very little understanding of what the deal was about and you worked all night, forwarding emails and proof-reading documents running into thousands of pages, with very little understanding of the transaction and the underlying law.
By way of example, I spent a six month seat in the litigation department, never having seen a Claim Form, never having made an application and knowing nothing about the practicalities of dealing with the Court (or arbitration tribunal). Conversely, when I moved to a regional, smaller firm, I was suddenly making applications, drafting Court documents and being given far greater client contact and opportunities to advise.
There is something to be said for having MC training - for some reason, it opens doors everywhere. But the hours are gruelling, the transactions can be extremely dull (I had no interest in securitisation or other complicated financial transactions) and there was very little law involved a lot of the time. If I had my time again, I'd have gone to a much smaller firm to begin with and done something I found more interesting (maybe clinical negligence or something useful/worthy), rather than following the 'normal' City route.
That said, some people do love it. A friend of mine is at a US firm, bills in excess of 2400 hours a year and loves his job (he's very lucky that he's a person that needs little sleep). I escaped to the regions and am far happier with a 9am - 6pm role, nice colleagues and decent money.