Pros
It is a large enough organisation to allow movement within different global businesses but to have completely different experiences (e.g. working in Retail Banking & Wealth Management vs. Commercial Banking vs. Global Private Banking vs. Global Banking & Markets). Significant soft skills training is offered to permanent staff. Good staff (be they perm or contractor) are retained and moved on to other projects or departments. Good perm staff are invested in and can climb the ladder quite quickly. Most departments are less "stuffy" and demanding than the American Investment Banks.
Cons
Bonuses are not so great in relation to annual profits and the American Investment Banks. Contractor rates are less than competitive banks (the "prestige" of having HSBC on your CV is deemed worth the difference).