Vantaggi
Great on the cv when it's time to find your next job. I worked at eBay for nearly 5 years and it really helps when it comes to looking for your next job. eBay is still seen as one of the top Internet companies to work for and is known for only hiring top talent so having it on your cv makes you stand out. Also, it's a very interesting business to be in, right in the intersection between ecommerce and social media. Some would argue it was one of the first "web 2.0" companies and is still exploring that space.
Svantaggi
BIG machine, loads of processes. As with all big companies there is a lot of processes at eBay to the extend that product development can take a very long time. If you the type that "just want to get things done" I'd argue that eBay is not for you.