Pros
They're an agency 'representing' the hiring company, acting as the HR for them. They are quite relaxed, will leave you alone, will not contact you if everything is ok. No strings attached
Cons
NO training provided at all. NO personal development plan, NO space to grow - as you're a contractor. They want you come and then finish and leave. Benefits virtually don't exist, minimal pension scheme in place (up to 2%) after 3 months, no childcare, no bonus, nothing at all. Have to accrue your own Bank Holidays and public holidays depending on how many weeks you've worked, otherwise you don't get paid for them. Confusing line management - you'll have to contact and report to your hiring manager in the establishment, AND the Adecco representative on site.