I personally have yet to see someone that looks good in a poncho.... when I see slender people wearing them, it just hides their nice figure and makes them look wider. When I see heavier women wearing them, while it may slightly hide the extra weight I still think it makes them look larger -- if you look horizontally across their chest or stomach area, you can't tell where their torso ends and their arms begin so it's like the width of their arms are being added to the width of their torso!
But really I think it's about the same effect as baggy clothes -- as Stacy and Clinton on "What Not to Wear" always say, baggy clothes make you look wider while structured, fitted (but not too tight) clothes make you look slimmer! (Or curvier, or whatever you want, depending on the exact style of the clothes.)
Sorry I am a TLC addict
I'm not trying to make any one mad who likes ponchos or baggy clothes, that's just my opinion!