She's a great actress. Half the time when I'm watching her show, I forget that what's happening is on tv. It's 'cause she's so great at conveying feeling in her own bizzare, screwball sort of way. She's one of the few actresses I know who can take one large, poor family facing all sorts of difficutlies...well, hilarious.
One thing is for sure: the fact that she is overweight just contributes to her unique character. She's impressive, really. She's what keeps the show Roseanne ticking (obviously), and I think she's the best person that coulda ever been picked for that part.