Avantages
Fantastic exposure to real responsibilities and fast paced work. In my first placement I was working flat out preparing for EU Exit, with exposure to senior ministers, real deliverable change and a great work environment. The level of work you are expected to do is of quite a high responsibility and can be challenging, but this is taken into account when reviewing you. Work life balance is great too, with plenty of opportunities for socialising outside work and building employee relations. The flexibility of working arrangements is also something that is highly valued by the CS, which is something I never saw in the private sector. There is a strong emphasis on wellbeing and self care, which is great to see as well.
Inconvénients
The quality of your fast stream placement does seem to be extremely luck of the draw, especially if you are a generalist. Although mine was fantastic as a Project Delivery specialist, I have heard some horror stories from other fast streamers of being stuck in dead end offices with no challenging work (some from PD specialists too!). Would recommend immediately trying to switch placements if this is the case, rather than sitting with it and moaning like many do. Pay is comparably low to other Civil Servants doing the same work, however you are fast tracked to a higher grade at the end of the scheme and often pipped to jump up even earlier if you have the initiative to pursue it, so IMO it balances out favourably in the long term.