You lot have made some great points. Let us remember that the avge consumer in the states has two considerations that drive the choice of car they drive: fuel economy and purchase price. The bargain basement buyers will never make Inifiti as popular as Ford or Chevrolet.
Even those who buy BMWs and Mercedes are buying them at least partially because of their pedigree earned over many many years. Infiniti is too new to even have a pedigree (20 yrs in US, less than 10 years in the UK) and due to it's completely inept marketing team, it's not likely that Infiniti will ever reach it's full potential for sales in any country.
In England, the M's that are most often plated (MOT testing) are deisel versions not even offered in the states. The M30 diesel is turbocharged, same body and interior as the M37 but with a 3L diesel engine, and gets surprising fuel economy (nearly 50mpg hwy, 30mpg city), but you won't see that model in the states - dunno why.
Also, in Europe, the M is seen as a 'luxury limo' due to the M37's near 45,000 pound price tag in UK and more subdued body style - curvy and smooth instead of agressive and sporty like other cars in it's price range. It's up against BMW, Audi, Jaguar, and Mercedes - all with a long pedigree of being proper sport saloons. For them, the sport handling is first, luxury is second. The M37 / M30d are seen as a car that puts luxury first, and driving excitement second. BTW, these are the only M's available in Europe - seeing an M56 is more than rare - it's not bloody likely unless it was shipped from outside Europe by a sheik or equivalent.
Here is an example of how the M is presented and sold in the UK. An old video by Simon (well known in Infiniti circles in Europe) from the Infiniti Center in Reading, driving an M37 petrol.
http://www.youtube.com/watch?v=4Tu5R3tN7tI
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A German reviewer driving the M30d gives some reasons why the M fails in Europe compared to the big 4 (Audi, BMW, Mercedes, and Jaguar)
http://www.youtube.com/watch?v=euG5G0IM5IM
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And finally, 5th gear's review of the M30d (about 40,000 pounds in UK):
http://www.youtube.com/watch?v=9MnMyObTMh0
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