I don't think the 360 is underachieving. I also don't think its overachieving either.
It's selling more than Xbox, has gained a small presence in Japan, and has expanded the brand well, particularly in US. It's also got a great library and sold many titles well.
I think the simple truth is, given the price of the lowest SKU, that the brand just doesn't have the acceptance or demand to gain WW sales like PS1/PS2.
The Wii for example still outsells the 360 WW with a higher base entry point while the PS3 only trails it a little WW despite a much higher entry point. At the price, and with its library, if the brand was good enough then the 360 should be selling more IMHO.
This is probably down to basic brand recognition, being seen as a shooter only console (erroneously) by many, RROD, minimal traction in Japan, the economy and many other effects.
However, I doubt MS are anything other than happy with progress so far. I think the LTD should be higher given the expenditure and effort on exclusives, etc. but perhaps I was expecting too much initially.
PS3 is of course a clear step down, at least so far, from where Sony was with PS1/PS2. Hopefully lessons were learnt. Given how close the WW sales are, Sony may close the gap yet, but MS will be fighting them the whole way (as is quite right in a competitive market).
Try to be reasonable... its easier than you think...
