Having read right through this thread im quite surprised how unbiased it is. Thought that on this site the Alfa would be seen as the be all and end all.
Although there are always exceptions to every rule (as demonstrated by the people who have posted about unreliable BMs) in every measurable way the BMW range (lets stick to 3s and 5s for now) are a better car. Better built, more reliable, better resale (if you pick the right engine), better dry handling or balance (most race cars are rear wheel drive for a reason - (rear wheel drive driven fast in the wet takes a more skilled driver admittedly)).
You only have to look at the various owners surveys on the net to see where BMW comes in relation to Alfa.
Overall running costs, in my experience, tend to be less with the BMWs as there are more specialist parts are service people around for BMWs. I have spent more on my 156 V6 Veloce in the 6 months I owned it than I spent on my previous 528i E39 in the 3 years I had it. Fuel consumption was better on my heavier (similar power 528i) too. The Alfa also seems to have the softest paint on any car I have owned!!
Our cars have problems with cambelts and Selespeeds. BMWs auto box (although not the same thing) is very reliable. Most (not all) six cylinder BMWs have camchains that last the life of the car. I sold mine with 170000 miles on the clock, smooth as silk, never had head off, never had box out and on original clutch.
Now as I said "in every measurable way". There is much more to owning a car than the things you can measure. I like the feel of my 156 better than the 528i. It feels more alive and has way more charactor. I like the unique looks and the comradery of Alfa ownership. But does that make it a better car than the 5 series???..... Almost certainly not. Does that worry me?..... Not one bit!!
Having said all that I will have to sell it soon - will let you know when if anyone wants a Rosso and tan leather (Momo) V6 Veloce with FSH, new belts, tensioners, f & rear discs and pads, 91000m- as its just proving too small. A 166 is not for me (too ugly sorry!). So will be looking at a BMW 5 or an A6 (whoever said that the Audi interior was rubbish clearly should not be driving with eyesight that bad - universally aclaimed as one of the highest quality interiors on the road today)
Whatever you drive, enjoy it.