They are pretty awful and aimed at people with little or no business experience.
As I understand it, they will charge 3 months gross for a contract and then charge 20% per month to invoice the clients and *cough* provide administrative support. On a £5000 contract, you would pay them £1250.00 to take on the contract and £83.00 per month thereafter giving you a first year outlay of £2250 to gain £2750.00 - the £500.00 you've earned would hopefully pay for your cleaning materials and equipment and travel expenses but would not be likely to be enough to employ someone to do the work for you.
The Global contract carries penalties for losing the contract through poor performance - it would be funny if it wasn't so exploitative.