There is still a major problem with using Microsoft's attachEvent, and that is that the this keyword does not work properly.
It works properly in Internet Explorer when using traditional event assignments, such as el.onclick = ...
But when using the attachEvent method, the this keyword instead refers to the window object, and is effectively broken.
You can get some information from the event object, but there can be major problems there too when the triggering element is different from the target element.
When someone else wrote an addEvent function that used attachEvent or addEventListener, the problems with attachEvent were delved into in this Quirksmode article on how addEvent is considered harmful
That inspired a war of sorts, where people tried all sorts of things to fix the problem, which resulted in a series of Rock Solid addEvent functions being created. One of which is Dustin Diaz's rock-solid addEvent function.