I have been using the 45mm Leica macro lens (Leica 45mm F2.8 DG Macro Elmarit Aspherical Mega O.I.S) for a few months now, so it was good opportunity to compare them with the same camera (Olympus E-PL2).
What I found was that I liked the Olympus more than the Leica 45mm lens. With the extra focal length (60mm v 45mm - which gives a narrower angle of view), I could stay a little further from subjects that don't like a lens pushed right in their face (and swim away), this was to me the big advantage over the Leica 45mm macro lens. Also the lens is cheaper (significantly), and doesn't need another port (the Leica 45mm does to work effectively).
Although I must admit the Leica lens feels like their is more metal and glass inside, the Olympus is quite light and I'm sure has some plastic in-place of metal inside and out! Image quality wise, well I am fairly critical, but I don't see any difference between them. I have checked Chromatic Abberation, Focus, zoomed at various sizes in Adobe Lightroom and I think it would only be a scientist who could pick the difference!
I've uploaded some photos taken with the 60mm lens (and will continue with more) to my Flickr Photostream, also check-out the Underwater Micro Four Thirds group on Flickr, where other photographers from around the World display their work with these camera's.
UPDATE AUGUST 2013: Olympus have now released a Focus gear for use with the 60mm Macro lens. Part Number PPZR-EP03 this rubber focus gear slips over the lens and allows the zoom control on the Olympus PEN housings (All models from PT-EP01 to PT-EP10) to operate the manual focus ring on the lens. A great addition for photographers using add-on wet lenses for ultra macro photography.