In my eyes it can't be considered a success.
Why do you think Microsoft entered the console business? To finish sixth out of 6 consoles this Christmas? Do you really believe that this was Microsoft's goal or that they are happy with being dead last currently in Sales? Yes, the PS3 is in a similar boat but at least there's an outside chance that its Sales might improve next year while the 360 now had 3 Christmas periods, therefore it's no longer a new console, by now it should have a much larger userbase than it has. And anybody who says 'Well, it's only their second console' forgets that other console makers had already huge success with their FIRST, let alone their second console
Microsoft is a million miles away from profit. Anybody who argues that every 360 now sold turns a profit for MS is very naive. It reminds me of those kids that say 'A CD costs 50 cents to manufacture so record companies turn a 10 Dollar profit on each CD'. Well, sure, if you ignore the costs of paying the artists, the song writers, the musicians, the technicians, the sound mixers, the marketing companies, the resellers, the background singers, the royalties, the rent for recording studios for several months, promotion etc. etc. So if you believe that the only costs associated with the 360 are the manufacturing costs then you're mising all the Billions in development costs and marketing costs and all the rest. Fact is, until the 8 Billion that MS have invested so far have come back in through Sales (Software contributes of course something to this), the XBox 360 is not profitable.
When the original XBox launched Bill Gates said in an interview that their goal was to sell 100 Million units. They ended up selling 25. MS even declared the XBox a failure in retrospect. So for the 360 to be a success it has to perform better than the XBox. And that doesn't mean 27 Million or 29 Million or 31 Million, it means MUCH more. As I said, Microsoft didn't enter the console business to finish dead last out of 6 consoles this Christmas.