The 50mm/2.8 macro is awesome. I think 50/55mm macros are underrated. People think they need a 90 or 100 so they don't frighten the bugs, or they think they'll block the light, but I still like my 55/3.5 better than my 100/4 macro. I've gotten lots of 1:1 shots of bugs, no problem with a little patience.
I like the shorter macro better because it's shorter and lighter at any given magnification, needing half the extension; easier to hand hold for the same reason; getting closer means less problem with leaves or other vegetation getting between lens and subject; and (obviously subjective), I like its perspective better at close range.
If I didn't already have a 55 macro I'd buy this 50/2.8 in an instant.