I couldn't help but LOL when I read Monica's diatribe :-) of Willoughby because I remember having the same feelings myself especially on my first reading and even to some extent on the second one. I'm sure I didn't detail it quite as well as Monica but the appall, indignation and incredulity I felt at the character Willoughby reveals during his speech to Elinor was similar to what Monica obviously felt. His audacious arrival, almost barging in where he was not wanted and then his excuse-ridden pleas for some exoneration only deteriorates his character further leaving even the reader feeling betrayed.

This is the third GR I have participated in for S&S and strangely with each read, perhaps because of prior knowledge, I find my reaction leans not as much toward reacting to this despiciable display but more toward trying to discern why he is like this and how he mangaged to (as have other JA characters) gain admittance into 'good' society. You know those old cartoon/old movie melodramas that you sometimes see on the classic movie channels where the hero/heroine are nauseatingly pure and the villans have no redeeming qualities? Well, JA to the best of my knowledge, gives faults or at least little imperfections to those with sterling character. By the same token the characters we 'love to hate' are not flat one demensional characters either with no positive features (for lack of a better term).
Willoughby oozes with wit, charm and personality. The weird thing is that his positive attributes actually add to his villany because he uses them to assume the appearance of goodness and of having the best of intentions.
I was under the impression that the men of the family and/or social circles were responsible for checking out the character of unknown men or at least keeping a watchful eye for character flaws that might alert them that something was amiss. Willoughby's 'gifts' allow him easy admission to the hopitality and respect of the Barton Park circle. Elinor is the only one who shows any skepticism because of the cavilier way that Willoughby degrades the Colonel but even she, I think, finds him otherwise fairly charming.
He does, IMHO, have moments of genuineness such as when he implores Mrs. Dashwood not to change the cottage. Even though he is speaking in terms of loving the cottage, I feel it is the home that the Dashwoods have made of it with their affections for each other, their humor and general good will, etc. that he is attracted to and finds appealing--probably something that he lacked when growing up and throughout his adult life. Unfortunately, the little snippets of humanness are practically rendered invisible in the wake of his ruthless and cruel behavior. I can only shake my head and think that he could have been so much more.