Pros
For the right occupation, Manpower may be able to keep you employed long term between several client companies. Work may be more interesting with an opportunity to travel or to get hired direct with a client company you like.
Cons
If you lose your temporary position for a few months, you lose all your built up benefits no matter how long you have worked in the past. Health Insurace is poor, but Major Medical is better the past few years after many complaints. You can end up in tong term emloyment with one client company working for wages without the great benefits, profit sharing, training they provide to their employees. I would call this a labor laundering practice that has become common among temp agencies. Client companies will often low ball your wage if they want to hire you direct.