Whatever your views of the man, it would be hard to characterise the Mark Penn era as a dull one for PR industry observers. During his six-year tenure as Burson-Marsteller CEO, the pollster oversaw more 'mini-scandals' than most, from his infamous work for Colombia during Hillary Clinton’s presidential campaign, to the firm’s ill-advised Facebook smear campaign. Along the way, MSNBC’s Rachel Maddow famously uttered the phrase: "When evil needs public relations, evil has Burson Marsteller on speed dial." For a big PR firm like Burson-Marsteller, those issues are probably par for the course. And it is worth noting that, under Penn, Burson became a genuinely big PR firm once again. The agency’s storied history at the pinnacle  of the global PR industry makes it easy to forget that Penn arrived at a fairly critical juncture in Burson’s history, becoming the third global CEO in less than a year after Thomas Nides’ six-month stint in charge, at a firm where revenues had shrunk to around $250m. By the time he leaves, Burson will have almost doubled in size to more than $450m, as detailed by Paul Holmes' extensive analysis in his scoop yesterday. Detractors will doubtless point to the inclusion of Penn’s PSB polling unit within those revenues, and question whether “pure” PR growth has rivalled its competitors. That may be a red herring; many of the larger PR networks have grown in recent years by diversifying into non-traditional areas. Penn made much of his desire to transform the PR industry - from the “golf break” to the “Blackberry break” as he once told me - and the integration of a credible research arm (however patchy that process was) was no bad thing. Neither did it escape the attention of his CEO peers, several of whom have attempted similar initiatives. Dissatisfaction with the status quo characterised much of Penn’s work at Burson. After enduring heavy media criticism in the aftermath of Hillary Clinton’s failed presidential bid, he rarely appeared troubled by the recriminations this approach might trigger. A pollster by trade, Penn did not always appear totally comfortable within the confines of the PR discipline, no matter how hard he tried to expand it. That discomfort may have rubbed off. A number of senior leaders left during the early years of his tenure. Penn refashioned the leadership in his own image, bringing in political heavyweights and attempting to force Burson into a more digital, analytics-focused future. There is a fine line between disruption and discord, and enough evidence to suggest that the cogs which power a global PR network had begun to creak a little less smoothly at Burson in recent months. Despite Penn’s worldwide political experience, attention appeared to favour the firm’s US homeland. Harold Burson’s heritage in pioneering the global model gave it an enviable geographic network, yet high-profile global account wins were not exactly frequent. Bob Pickard worked tirelessly to revive Asia, while Jeremy Galbraith presided over a quiet renaissance in Europe, the Middle East and Africa. New CEO Don Baer inherits a network with strength in all of the key global markets. After six years, it is probably unsurprising that Penn hankered for a new role. A WPP move may have been on the cards; ultimately, his arrival at Microsoft gives him an opportunity to help a company that he knows intimately. His successor, stressed WPP CEO Martin Sorrell, “is collegial with good people skills”. Penn may not have have shone in that regard, but it is difficult to argue that he leaves Burson in bad shape to meet the future.