Vantaggi
Most people are friendly, there is a nice atmosphere - much less aggressive than most other trading floors. The benefits are pretty good and competitive with what other banks are offering (good pension, holidays, private medical care). The gym in the London HQ is very nice for a cost of 25-30% of what you'd pay externally for a decent gym. There is a good work-life balance, even in Global Banking and Markets (the investment banking division). HSBC is slow to make massive redundancies, so there is more job security in tough times. Overall, if you aren't concerned about developing your career or making serious money, then this is a pleasant and easy place to work with decent base salaries and good benefits.
Svantaggi
This is not really an investment bank, it's rather more as I would imagine it would be working at British Telecom or British Gas. Extremely conservative organisation - you will not get ahead by rocking the boat or trying to do something in a new or different way (this is in fact one of the few things that might get you sacked). In order to get approval to do anything, you have to go through about 3 or 4 committees. There is very little encouragement to develop your career or move up in the bank - you have to fight your way up and this is nearly impossible if you are not a white British man. The bonuses are shockingly low compared to what competitors are paying (in Capital Markets). Very few people, particularly in senior management that you can look up to - most of them just seem to be bureaucrats. In general, there are a lot people who don't do much work, but sit around from 9am-4pm and collect a salary (they were probably ambitious at some point until HSBC crushed it out of them). There is very little incentive to put in a lot of effort - it will not be rewarded.