poly batting comes in different lofts. so if you like to show off your stitching, you might go for a high loft. if you want a flatter look, there are low lofts available.
they weigh a lot less and some people think they're warmer for the weight, because they do hold in body heat (they don't breathe). every cheapo quilt i ever had had the cheapest poly batting in the world and it never melted in the dryer.
if you like the puckery look of a cotton quilt that's been washed, poly won't do that. the polyester won't shrink enough to pucker. and you should preshrink the fabric before you use it, so the fabric doesn't shrink later and pull the poly up with it. but there's nothing wrong with it. it all depends on the look you want and the feel you like.