Pros
Great place to learn how to do Internet things. It is really great to work on something with potential to influent hundred of millions Internet users. And there are many workaholic guys and really smart guys working here. You can find many great technology around here.
Cons
Yahoo is a big company operated like hundreds of small startups. However, it doesn't have the startup efficiency but with startup workload and big company's politics. The managers I met seem tried to manage the team like a startup but without a startup manager's technical skill and also without a traditional manager's people management skill. So many layer of management and the people involved in a project are so many and hard to get project going smoothly. Also, poor documentation and it is hard to get know what in a document is in production as well as valid for your situation. And the most important thing for people work here smoothly and favorably is social and politics. So many temp project and incomplete project around and so many changes for project.