Pros
If you have just left school or college and you aren't sure which direction you want to go in then 02 is probably a pretty decent start, you get a reasonable staff discount of mobile phones and the call centers are generally kitted out with high end distractions; x-boxes, pool tables and big screen tv's, plus free phone lines and internet access are all put on for your leisure time.
Cons
the downsides far far outway x-boxes and internet. Middle management is generally picked from staff who have a desire to be management, thats it, no qualifications needed. one minute they might be sat next to you as an equal then a crash course in NLP based management tactics later and they are declining your holiday leave left right and center. O2 would like to think of itself as the communications industries answer to google, but all the tennis ball catching chocolate cake fuelled team briefings and strategically seated 121's can not make up for over idealistic senior management incompitent middle management and a staff turnover that makes mcdonalds look like british leyland in the 70's. Getting holidays and time off approved is a lottery, with many people trying to book a two week holiday and being told they have to work for two days in the middle or face a disciplinary, try that whilst on a two week holiday in crete! The pay is below the national average dispite what the dogma tells you, and this is all under the guidance of the union, who amazingly managed to succesfully negotiate a pay cut for thousands of employees during one pay dispute.