The Olympus lenses that Phate has are a bit bigger in size and weight though, and lack IS due to Olympus' choice to have IBIS.
The IS is built into the body isn't it? I've definitely noticed it kicking in when in use, that's for sure. I think it's more necessary on smaller and lighter camera gear.
Having been a photographer for years and used DSLR's for years including professionally and now having had my m4/3 for over a year and used it daily for 8 months straight whilst backpacking my thoughts on comparisons are as follows.
IQ - unless you're pixel peeping, with decent glass you will REALLY struggle to find a difference. The 5D3 had a better DR than the Oly but that's not to say the Oly is bad. Just that the 5D3 is exceptional.
DOF - Whilst m4/3 is good FF is definitely better. m4/3 has to be worked differently to get the most out of the DOF but it can and does provide excellent results. Just like FF, primes is where this will shine. I have a 17mm f/1.8 which is the answer to Canons 35mm f/1.4L and ignoring the size and weight difference (because they're stone cold facts, regardless of opinion) the Canon will obviously deliver a more creamy bokeh but the Olympus doesn't exactly lack in this department either. See my below examples.
Size & Weight - yes it does make a difference. Simply put. I'm not being biased. Yes I wouldn't mind owning a DSLR again now I'm not moving around as much but as I do not work professionally with photography anymore and as the quality delivered by the olympus isn't lacking for me in any way then simply put why would I lay out thousands of € just to have a DSLR again?
To give another example, when I picked up the prime lens in Tokyo I genuinely didn't notice the difference in my backpack. It's that small and light.
If I started shooting weddings or advertising services again, then it would be no question.
m4/3 cameras are also more friendly to non-photographers. My girlfriend before really didn't want to use the 5D3 or even pick it up. She was scared she would break it! Despite it probably being more robust than the Olympus. But the m4/3 she will pick up and use, experiment with and have fun with. If I take it to a party it will get passed around and my friends will give it a go and we'll all have fun. If I took the DSLR people genuinely didn't want to go near it.
The main thing that bugs me about the Olympus is the battery. 2-300 shots and its dead. A big difference coming from a shot capacity of 3000+ on the DSLR. But I'm looking into a grip anyway.
Anyway, there is my further 2p