Avantages
I worked at one of the Belgian branches of Altran. I started with great hopes and promises but quickly it turned out to be another body shop who views itself in the giant's mirror. Altran business managers present it as the ultimate design expertise center but it is just another temp hire business, particularly in Belgium. Maybe a good starting experience for someone as you might get to work in differnet sectors and roles, but you gotta have the eye and motivation to actively seek out for it. So be smart and use it as a stepping stone, like many do.
Inconvénients
Once you are on a contract you are there until the client no longer needs you. You have the carry the heaviest burden but you find yourself out budgeted for the celebration party, i.e. you are already out once things seme to go in the right path. Otherwise a consultant may get hired to shift the blame after it is clear the project is going nowhere. For business managers the consultants are the commodity, valuable only when they are on a paid contract. Your salary is a direct loss on your manager's P&L, so many broken promises. Forget training or advancement unless you seek out yourself, make your client pay at least half of it and promise to increase your rate to compensate. As in any consulting, if you are experienced you find yourself very expensive for the client rapidly.