Gotta update my profile! I loved the 550d and used it for years. I bought a used S&S housing, dome port, macro port, and lenses. But the overall physical size of the whole system just started to feel really burdensome for travel. So I sold everything, sold some other things on Ebay, and got the EM10 and a Nauticam hosuing. I kept the D1 strobes. I basically have the kit lens and then the Panasonic 8mm fisheye and the Olympus 60mm macro. But my main interest if macro. Overall, the whole system is so much lighter, smaller, and easier to deal with. I know that it's never going to be 100% of the quality capable with a full blown DSLR setup, but I have a long way to go before my skill level will exceed the EM10's capabilities.
I shot everything in RAW, so it was all in the 4/3 ratio. I decided to crop some of them to 3/2 just because it's easier for some prints and some of the crops looked better that way. I really enjoy the EM10 and have no regrets, though I do miss my trusty 550d on land.