... As far as I am concerned, the 45mm 2.8 Lumix is a wonderful lens. It is sharp, handles well, and doubles as an excellent portrait lens. ...
I would agree fully with Antonio, but with a (big) catch. I found that the autofocus on the E-P2 was unreliable: slow and iffy. It tended to hunt for a while, even when the focus range was on the Limited setting, and sometimes stubbornly refused to settle on a focus at all. I never really warmed up to using it as a manual focus lens on the E-P2, since the art of focus-by-wire seemed to elude me.
For my part, I had kind of given up on using it either as a macro lens (I preferred my old OM 50/3.5 macro, which I first purchased in the early 80's) or as a portrait lens (I preferred an OM 50/1.4, which I picked up for a reasonable price on eBay). I still liked it as a medium-reach tele lens, and used it a lot as a walking-about lens. (Not a surprise, since my favorite focal lengths in the old film days were 85mm and 100mm, and this one--at 90mm equivalent--is smack in between those.) It's undeniably clean and sharp, which is why I hung on to it.
All this changed, I need to add, when I switched to a G2. Suddenly the lens came into its own, with the autofocus snapping quickly and surely onto the target. A quick touch on the screen will also let me switch to a new target, and it seems to go there without any hunting at all. To my surprise, even manual focus is more comfortable for this lens on the G2, with the camera popping up a focus range indicator that reinforces the focus-by-wire movements.
I realize that you're asking about the E-P1, though. As I recall, Antonio also has an E-P1, so he'll probably be able to give you a better feedback on how this lens will work with your camera.