Vantaggi
If you are beginning your career, there are few other companies that can teach you how to build world class software. (across consumer or enterprise, retail versus online services). There's such a wealth of knowledge in this company, you will grow your career experience very quickly that will be hard for others to match. (access to MSR, industry architects, strategic thinkers) The benefits and salary are very competitive - get several good reviews at the more senior/principle bands and you'll love the stock program. The campus culture in Redmond is still very cool (although there once were random arcade machines between buildings) - beats any major city corporate culture on any day.
Svantaggi
There's an issue with key talent bleeding out of the company, and potentially strategic direction. This raises questions if you think your career is best parked at this company in the long run. RE: key talent, this covers execs that have left (publicly), but also applies to key contributors at the most senior levels (i.e. the company has trouble hiring and retaining externals to fill strategic roles, only seems to be able to promote from within). Re: strategic direction, investors publicly call out that Microsoft's story around Windows/Office has only so many years left in it. Yet practically no major business unit has succeeded or shown promise to take the Windows/Office mantel. (think if investments in mobile, online music, online TV, tablets, social networking, search (w/o Yahoo), and yes even Xbox have materially changed the game for the company?) Said differently: You get to build products with the best, they just may not be the best products.