Having owned both the R8*V8 and 2 ///M3's, I felt compelled to share my thoughts regarding the "comparison" and the comments made thus far.
I came directly from a 2011.5 E90 ///M3 DCT, have previously owned an E46 ///M3 (6spd) as well as a 335is, X5, etc. I've been thru the BMW lineup, I consider myself an enthusiast and have driven track events, canyons, the occasional fun run on the streets etc. When it comes to a blend of overall street-able "performance" IE: Straight Line, Handling, actual real-world use - the ///M does a magnificent job of doing all of these things VERY well. Some say to the same level or better than the R8*V8. But as someone mentioned, the same could be said about the ///M3 compared to a Mustang or a Vette (just typing that makes me barf a little in my mouth)
In my opinion, what's missing is the intangible factor. It means different things for all of us, but for me it's about what excites you when you open up the garage. For some, it may be a Type-R - for others, it may be an Aventador. This is where the R8 beats the ///M3 hands down for me.
If I had a dollar for everytime I saw an M3 here in SoCal...well, I'd be driving an Aventador LOL.
To be honest, if I wanted a car that was fast as hell, full performance oriented and seated 4, it would be a GT-R, not an ///M3. But to me they're butt ugly compared to the ///M.
As a BMW enthusiast, it's great to be proud of the performance of your car and I understand that. But you have to take into consideration all the factors if you're a true automotive enthusiast, regardless of make/model (unless it's american LOL /kidding) - it's easy to play the numbers game, the seat of the pants horsepower, etc. There will always be a faster, better car - enjoy your car and stop worrying about comparisons. Appreciate what other cars have to offer as well or better yet, own both or all.