Pros
Somebody with very little or no experience can start at the bottom of the ladder here and progress quickly. Although usually you will start on close to minimum wage with an agency, due to the generally low quality of recruits, you can work at an average to hard rate and easily impress management. After working for the agency for 4-6 weeks I was offered a full time job to train to become a class 1 driver (fully paid for). As a mate I think the salary is 16k however drivers starting salary is 23k. This is very appealing given that you could potentially have no GCSEs or A levels and be on close to the national median salary in less than a year. On another note the work is not too tedious because every day will involve being in a different are (Oxford, Birmingham, Bristol etc)
Cons
The early starts (3:30-7:30 most frequently) and the flexible shift times; one day you may start at 07:30 and finish at 13:00 and get paid a full day regardless, on other days you can start at 03:45 and finish at 18:00 and still only get paid a say's work. The work is also VERY physical; moving the cargo as a beginner is not easy unless you are very strong. Eventually you will master the technique and strength becomes less relevant although this will take a long time.