Pros
FDM is great for grads and career changers without IT experience (I can't speak for Ex-Forces or Returners) who wish to apply themselves in an IT career in top companies that might otherwise be difficult to get into.
Cons
When you start working at FDM (as a grad) you are required to be geo-flexible within the UK and you are obliged to work on behalf of FDM for two years on a below market salary. Both of these are to be expected and not really a con, as you are told about it before signing the dotted line. However, beware of pushy account managers who try to push you into any job whether or not it is aligned with your skills or aspirations. I've had a couple of clashes with account managers that have tried to compel me into going for admin heavy roles despite my previous experience and desire to upskill. Essentially, it appears that their reward system is set up so that they get something for putting you in a role no matter your expressed preferences or whether this role helps you to develop career wise. I would also say that once on site, you are essentially left to get on with it. If you find yourself in a difficult placement, you may find it hard to get out of it, unless your contract ends or there are some other mitigating circumstances.