Pros: At this point in my life, the main benefit would be with women. Black guy, white girl poses little problem, while white guy, black girl is difficult [1]. Honestly, this dwarfs everything else.
Affirmative action might have been helpful when I was in school, though it's probably irrelevant to my life now.
Most ethnic groups besides whites have secret networks unavailable to white guys. Their quality tends to be lower than more focused networks (random HN reader or random lisp user > random black guy), but they are still useful.
In the future, the prospect of selling to government under the WOB and MOB (woman/minority owned business) quotas might be useful, though I doubt I'll actually get into that sort of business.
Cons: Racial profiling. Since I rarely shop or drive, this isn't much of an issue for me.
More difficult to live in the still-racist parts of the country. Even as a white guy, I'd like to avoid them.
The main reason I answer so readily is that being white isn't a significant part of my identity. So it would take only a very minor perk to make me change.
[1] This only applies to black American women, African women don't seem to have any real problem with white guys.
[edit: genuinely curious why I'm being downmodded.]