Best saloon i car i have EVER driven to date is a BMW M5. Nothing compared to driving that beast for a year.
What's interesting about that statement is that in a way I'd agree, not driven the current V10 M5 but have the previous V8. And yet I chose the E (like you did) over a 5 for my everyday, economical, cheap to run daily driver.
The M5 is very unlike a standard 5 series, for a start there's no smooth auto and no smooth ride, but the feel, enjoyment and handling are amazing when you're driving it hard, and therein lies the rub.
99% of the time spend in a car is not spent throwing it around bends with smoke pouring from the rear tyres. Journey times in the UK have very little to do with feel, handling and power. Comfort, style, image and ergonomics matter more, and the for most people the E class beats the 5 series on all four counts, the C beats the 3 series on all four counts and the S beats the 7 on all four counts.
That Mercedes is now beating BMW on reliability and customer experience speaks volumes. When I went looking for a car with £400/mth and an A class to trade in my two nearest BMW dealers showed no interest, my two nearest Mercedes dealers where up there with the follow up calls, test drives, moving cars between showrooms for me etc, as were my local Volvo dealer. BMW did not get my business.