The A342/3 were very good aircraft in their time. Airbus gambled that airlines would be uncomfortable with a wholly twin engine long haul fleet so gave airlines a choice of 2 or 4 engines with a high degree of commonality and a common crew rating. As it turned out safety issues with twins became less and less of an issue. The A330 had a limited range but excellent economics if you didn't need sectors of over 12 hrs. They are still selling well and a re-engine version is likely to be around for a good while.
Airbus made a mistake with the 345/6. The weight is a third more than the 343 and the fuel cost per seat mile is no better (an expert might be able to give precise data). The inflexibility of larger aircraft should be offset by cheaper costs per seat mile (or km). Airbus assumed cheap fuel. SIN - EWR nonstop worked with oil at $30 / barrel. The 77W had far superior economics and sales reflected this.
Was the A330/340 programme a success? Definitely yes. Was the 345/6 development a success? Probably the biggest turkey Airbus have produced. (Lets say the jury is out on the 380.)
The A321 is lighter than the 757 so cheaper to operate but with a shorter range. This is being remedied to some extent by the neo. It has strong competition from the 737-900 but don't overlook the advantages of a wider cabin. I read somewhere, I think anna-aero, that it is becoming the most popular member of the 320 family. As misd-agin says, the order book speaks for itself.
My two penny worth from a commercial viewpoint. I'll leave piloting and cabin crew comments to the experts.
Last edited by Peter47; 16th Aug 2014 at 19:09.