Pros
Yahoo has great employees, especially in the search marketing department. I learned a great deal from my peers and managers and basically everyone was willing to take time out of their busy day to help educate a new person. I found the teams I worked with willing to share the kudos for a job will done and willing to help out when projects were in trouble. Yahoo also has a enormous user base which allows for fast testing of ideas and doesn't make the company reliant on any one product or service. Finally, Yahoo has a great campus in Sunnyvale and the actual working environment is enjoyable.
Cons
Yahoo's technology is old and is constantly in catchup mode. Google, Facebook, etc passed Yahoo and have never looked back. Because Yahoo is always playing catchup, to get ahead they need massive releases that never fully work out. Playing from behind also drives product releases to happen before the product and services are fully ready. Yahoo also suffers from too many cooks in the kitchen and poor integration of past mergers. For instance, the search and display sales teams spent the first 3 years apart after Yahoo acquired Overture. The technology is still not integrated. This put Yahoo at a disadvantage when the combination of these assets should have been an advantage.