I own this lens and, by happy accident, ALSO own the 45-200 Panasonic. This lens is lighter, smaller, easier to handle, and at least as sharp at 150 as the Panasonic at 150. The Panasonic, however, is the size of a DSLR consumer zoom, such as the Nikkor 55-200 and not very compact at all. It handles like a DSLR lens, so negates some of the convenience factor of the m4/3 system. If you have an Oly EP/L/M-x or Panasonic GF3 compact body, the 45-200 will not balance well on it. If you have an Oly OM-D or Panasonic GH2, G3, or G5 body, the 45-200 will balance well. I owned the 45-200 first, found it awkward on my Oly EPL-1 body, bought the Oly 40-150, and now hardly use the 45-200. Hope this helps.
I have the G2, not sure what the diff is from a GH2 in body size.