CS... since you know I picked up the M3 as my DD, my sentiments are mirrored by some above. I will however add this - if you want to keep your M3 as more the 'precious' vehicle you don't want to tack day-to-day mileage and or wear - I've got some suggestions for DDs. I went through this the past 4-5 years when I had 'parked' my S4 for sale (and when I had moved to Capitol Hill for almost a year - would you really want to park a nice car on the street in DC?). I bought a crew cab Nissan Frontier LE... really nice having a 4 door pickup... for all the things you can't do with the M3 or any other space-challenged vehicle - also once you become a home owner, you want that bed (or if you own a motorcycle); then I picked up a Infiniti FX45 (representing a balance of some practicality, but still offering maximum performance for the class) - it was the highest performance crossover back in '04 (only to be outdone by the Cayenne Turbo); then I moved onto a X5 4.8is - thinking I wanted a bit more 'luxury'. I was mistaken with the last one, granted the 4.8is was a nice ride, it's awful on gas, and feels like you're driving a tank. I would highly recommend the FX45/35 if you can find something in the mid-teens - it truly does have lots of agile dynamics, while offering you ease of either carrying people or cargo, and fairly high in luxury as well.
Current stable: 2013 GT-R Black Edition, 2012 Audi A3, 2012 Frontier Crew Cab Pro4X. (recent: E92 2010 Jerez Black M3 6MT, 2008 Corvette Z06, 2005 Ducati 999bp, 2006 X5 4.8is, 2005 FX45, 2005 S4, 1999 540i6)