Vantaggi
ERM employs some great people and this really makes the place. Most of the partners are very friendly and the company as a whole can be very supportive (they certainly were for me, however I've heard differently from other people). I've had the opportunity to work on some really fascinating projects and get a lot of client facing time despite my junior level (contrary to what some other reviews suggest). If you put the effort into "selling" yourself within the company and pushing for opportunities to work on different things it's a really great place to work, however people who are less forward seem to have had a much worse experience.
Svantaggi
Massive bureaucracy at upper levels can make doing anything difficult. Some senior partners are just old boys who seem to do little more than keep their seat warm (however others are really good and hopefully symptomatic of a "new breed"). Internal politics can be a nightmare with each partner effectively sitting on their own personal fiefdom - some partners make everything easy but others seem deliberately obstructive. Pay rises are paltry and you'll quickly find your salary is waaaaaaaaaay below industry standard and you have to fight hard to get anything like a fair paycheck. If you don't have support from one or more partners then forget it. Getting a promotion can be tough unless you have a partner on your side to fight your case. Company does not have consistent process and systems across regions. Attempts to correct this seem to make things worse rather than better. Chargeout rates are much higher than competitors, which given the lower salaries is a bit of an irritant. Management claim the increased rates are because ERM is better than it's competitors, which is fundamentally untrue because there are plenty of excellent consultants working at other consultancies (that's not to say that ERM is worse than its competitors of course).