Noise is a funny topic. I still own a 5DkII, and also own a phase one back. My main setup now is an Olympus E-M1. There is a slight...not noise...but texture, very slight at all ISOs. This a m43 thing. However, real noise is very minimal. Even up to 5000 ISO. I have printed to 30" with no problem, even from ISO 5000 and i get excellent results. Upsized, and non- upsized. It's a matter of careful prep. I do very different sharpening with the E-M1 than with other cameras. I do much less. That being said, if you are upsizing, I highly recommend Alien Skin Blow Up as the means of resizing. It is much better than photoshop or ACR in my opinion if you need large prints.
I did a shoot for a client the other day that has a lot of moderately dark areas, and then selectively lit sections. ISO 400 was used with a 1/4 second ambient exposure to get the mood before the strobes came in. You won't believe how much noise is in the photo...and this is a full frame DSLR. I can get excellent clean results even from high ISO on m43. Again, this is pixel peeping. This is noise that won't even show up in a print. And if it does...the viewer is a bit too close for the size.
M43 can fully deliver however. E-M5 was the first generation that was decent however. Everything before that was a bit hard to live with. All the latest stuff is excellent though. Especially with the good glass.