2007: 11 wickets at 26
2008: 63 wickets at 29
2009: 63 wickets at 27
2010: 40 wickets at 34
2011: 13 wickets at 57
2012: 12 wickets at 20
2013: 34 wickets at 18
2014: 47 wickets at 23
2015: 28 wickets at 35
In the 9 or so years that Mitchell Johnson has been playing Test cricket he has had 2 stella years, 4 good years, 2 mediocre ones, and a shocker.
Since he averages 25 in both Australia and South Africa, but 40 on the subcontinent, and 36 in England it seems reasonable to assume that he hasn't always done well on pitches that don't bounce much. Not surprising really.
Overall, I would say that Johnson has been a valuable spearhead to Australia's attack, and one of our all-time best. Although, he is still a long way off being comparable to the likes of Lindwall, Lillee, or McGrath.