Pros
Potential to be placed at a good client including most investment banks, however you are not guaranteed a good placement, there are some really crap ones too. If you really can't get a job and will take anything then this is probably for you, if you can make it through the unpaid training. Not guaranteed a job afterwards though.
Cons
Where to start... - The training (if you can call it that) is terrible. You might get lucky and get a good trainer but chances are you'll be taught by an ex trainee who couldn't get a job so was forced to teach. - The whole running of the company is pretty corrupt and you'll feel powerless after you sign the contract. Unless you can afford £20k... - The sales team who find you the jobs and manage client relationships care about one thing, money. Trainees are literally money to them. - Very unprofessional management and if you do something to annoy them, like reject opportunities, they'll do whatever they want to mess with you. - It's quite likely that you'll get a job which is unrelated to what you went in for, though you may get lucky. - They try to make out like it's really professional, but it just isn't.