Avantages
As both someone who has been promoted and promoted others, I've found that good folks get promoted up to a certain level based on the merits of their work. eBay obviously has nice facilities, cafeteria and all that. And I personally love the mission of PayPal, the branch I work for -- to be a global currency.
Inconvénients
Once you're promoted past a certain level, promotions tend to be much more political. Probably not something exclusive to eBay, but it's not a great thing to experience. Also, frequently, someone of questionable merit gets promoted and that compromises the integrity of the process. Warring factions sometime create lack of role clarity which leads to a tremendous waste of time, time that could be spent on moving the business forward. Lastly, there's a significant trend toward "alpha male" executives which feeds the "warring factions" feeling.