Pros
Working at Auto Trader provides a fast pace every day and week. Months can go by quickly. There are always opportunities to step up and participate in new assignments and challenges, providing continual professional development and growth. Management supports taking personal time when you need it, and is supportive of taking time out for training as well. Their compensation and benefits are great and they invest in community service. If you make a good pitch, they may even support your favorite cause as well! In summary - they pay well, they care about you as a person, provide you opportunities to grow and time moves fast. Oh, and most of the people are very nice to work with and know what they are doing.
Cons
Not all departments or functions are created and treated equally. Some folks would disagree with my "best reasons" because of the role or department they are in. Some have less of the best reasons than my area, and some have it even better. This place does not move slow, priorities can change quickly/daily and if you are uncomfortable with a rapid pace you might feel overwhelmed. While this makes the day move fast, it also means that communications could be much better because there are so many suprises. While you can get great experience working on new opportunities that does not mean you would actually be recognized and promoted beyond Manager level. Typically executive positions are filled externally as they attempt to bring in fresh ideas and professional networks. As a result, large reorganizations at the executive level happen frequently and in some areas it is difficult to establish a solid relationship with your department head because he/she just came in and will be moved elsewhere within a year or less. People often move to a new desk/building (to sit near their new boss), don't bother unpacking unless you've sat still for a few months.