HI I came the other way to Oly E520. I was using a Nikon F4 and Aquatica housing and was worried that the Oly would not be good enough. I bought a system a year ago after Phil Rudin gave me some very sound advice.
I have taken it on about 4 dive trips in the year and run off about a 1000 underwater exposures and I have to say that it is not just an intro into DSLR (Sorry Jack) but a system that is capable of producing images of a similar quality to anything that Nikon or Canon can produce IMHO. It is mainly to do with the lens and I use a 50mm macro mostly and occasionally the 14-42 which is a great lens as well.
The system is light and easy to assemble the PT05 covers both of the lens I use, so the airline carry on is not so heavy.
I plan to run this system until it runs no more or I dont, can't see any reason to upgrade.
I have tried the live view underwater just for fun and it works OK except moving items but no real reason to use it anyway. The view finder is bright and surprisingly good, this is the one part I thought I would have to make a compromise on but I dont miss the DA-20 large viewfinder I had on the Nikon.
Nikon and Canon are fine systems and I spent 6 months looking at the Nikon solutions as I owned all the lenses. I cannot imagine why any non professional underwater photographer would not find this system ideal and the savings of about $3-7k is significant.
Lets hope the Nikon, Canon one eyes dont jump on this.