Pair programming is a practice in which two programmers work collaboratively at one computer on the same design, algorithm, or code. Prior research on pair programming has primarily focused on its evaluation in academic settings. There has been limited evidence on the use, problems and benefits, partner selection, and the general perceptions towards pair programming in industrial settings. In this paper we report on a longitudinal evaluation of pair programming at Microsoft Corporation. We find from the results of a survey sent to a randomly selected 10% of engineers at Microsoft that 22% pair program or have pair programmed in the past. Using qualitative analysis, we performed a large-scale card sort to group the various benefits and problems of pair programming. The biggest perceived benefits of pair programming were the introduction of fewer bugs, spreading code understanding, and producing overall higher quality code. The top problems were cost-efficiency, (work time) scheduling problems, and personality conflicts. Most engineers preferred a partner who had complementary skills to their own, who was flexible and had good communication skills.