Pros
I had the opportunity to work at Tandem for about 18 months, initially as part of a structured internship, which then led to a permanent role within the company. I found their commitment to inhouse professional development exceptional as I ultimately started with zero knowledge of digital marketing (my MSc Marketing was poorly lacking in this area) and left with a wide range of experience in PPC, analytics, social media campaigns, html and overall account management. I was supported through Google Adwords certification and a range of other training, which has been key to securing further marketing roles as my career has progressed. I also got a lot of face-to-face time with both clients and Senior Management which was really motivating as it meant that I felt very involved with all areas of the business. There was very much the attitude that you can define your own place within the company and the opportunities are there for those who are willing to work for it.
Cons
Like any decent agency you are expected to give 110% to meet client demands, so if you are a clock-watcher this isn't the place for you.