"Regular" (i.e. OM series and other) lenses do work, but there are restrictions. Manual focus, stop-down metering, and not all apertures are give optimum performance. Not all OM-series Zuikos are "good" on a 4/3 body, but some, such as the 28, 35, 50, 50macro, 90macro, 100/2.8 and 135 are very nice indeed.
Yes, it's a 2x crop factor, which is often a minus, but sometimes a plus. The biggest hole in the E-series system is the lack of fast primes. There are only a couple out. My major concern lens-wise was that some reported the 21/2 didn't do that well. But I saw shots from a member that gave me hope that it wasn't so. On an E-x it would be a 42/2, a really nice focal length.