Pros
Exciting pace of change keeps things interesting. The office environment is fun and the culture is relaxed. Work can be interesting and the impact is immediate. Skype's international network of offices provides for frequent travel.
Cons
Estonian culture is very insular and I have not observed foreigners in Tallinn office being promoted (foreigners probably make up 10% of Tallinn workforce). No formal review process exists and management seems arrogant and abrasive. High-level communications is pathetically spartan: Skype employees know to hear news about company from Techcrunch, not from senior management. Salaries are low, even by Estonian IT standards, and quality of people coming into organization is also low given ease of local talent moving abroad within EU and foreign talent not being attracted to life in Tallinn. Very little support given to foreigners moving to Estonia, especially compared to other IT companies in Estonia recruiting from abroad. Management has "take it or leave it" philosophy toward retention, which is relic of times passed when Skype was premier employer in Estonia. Bureaucracy in Tallinn office is a nightmare. Cronyism in Tallinn office is a serious problem, and employees who were hired early on have been promoted to very senior positions and kept on payroll despite their contributions to the company being questionable.