Firstly, I'd like to congratulate Stuart MacGill for making the tough call and retiring. IMO, he retired at the right time. He was bowling utter rubbish; it's not like his presence would have helped matters in the longe term, anyway. 200 Test wickets, from 44 Tests (despite his minnow-bashing) is, indeed, an impressive achievement, although he will forever be consigned to the role of 'Shane Warne's understudy' within the dusty confines of history.
I did feel that he was underrated by some in Australia - on this forum, even. In his favour, he could spin the ball as prodigiously as anyone. He had a very good googly to call upon and an OK topspinner. As pointed out, he also underperformed Shane Warne frequently when the two were bowling in tandem. Out of all of his performances, his 9 wickets against the World XI, IMO, will forever stand out, as will his 5-fer against Pakistan earlier that year (that legbreak to dismiss Mohammad Yousuf deserves to be a defining image).
I also felt that he was underrated as a batsman - during the 1998-2001 period and the 2005-2006 period, he could come up with some vital runs, ungainly though he looked (Adelaide 2005, Sydney 2006).
His flaws were a lack of control, both with the ball and on the field. He also lacked variation - he had nothing beyond a legbreak, googly and topspinner and also failed to vary his position on the popping crease. His fielding also wasn't particularly good (in fact, it was poor by Australian standards), although it would not have attracted notice in lesser fielding sides (i.e - 1990's Pakistan, the current Indian Test team).
I wish him luck for the future.
As for the Beau Casson debate, I'd be reluctant to play him - particularly in India. Few of their batsmen are fazed by the turning ball. India has been referred to as the tour of the 'dot ball' - a fair assessment, for mine. From what I've heard, Casson doesn't appear capable of delivering enough of those all-important dot balls.