Take this with a grain of salt as everyone's experience differs, but my wife owned an 06 E55, purchased it new and sold it 3 years later. It was never taken to the dealership other than routine maintenance. The only reason she sold it was it was running out of warranty and she got a ridiculous offer by a buyer after she exported it to Korea.
It felt very solid, everything felt substatial and the car had presence (still does). Handling was vague and questionable as it was for any W211 (this was somewhat corrected with the E63 that came out the next year), but it was extremely comfortable. The torque was absolutely ridiculous.
I'd peruse mbworld.org to get a sense of maintenance costs/reliability, etc.
For what it's worth, I think the C55 looks pretty nice.
If you can swing it, I'd go with the 07 E63 if possible.