Updates: Status: Accepted Comment #1 on issue 2057 by asmeurer: Units do not work together with evalf? http://code.google.com/p/sympy/issues/detail?id=2057
That is definitely a bug; it should just work. I am guessing that the reason that evalf has a subs keyword argument is that it is more numerically stable and/or more efficient than computing expr.subs().evalf(). But either way, that would have to be the workaround, until this is fixed.
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