The 321 is a European beauty. Great for short hauls and packing in lots of peeps. It is not a competitor with the 757. However, I hear the NEO, with improved performance, might make it more popular in the US.

Having flown one in a previous life, I must say it was definitely an after thought to the 319 and 320. At some point a manufacturer needs to say, "Maybe we need to design something new rather than stretch this thing into obsolescence."