pngs are nicer than gifs because they have true alpha blending, not just transparency. However, they don't work in IE. I'm using the "pngfix.js" script to make alpha blending work in IE, but it's just not perfect. The divs that the script adds mess up my CSS. True alpha blending becomes important if you're rendering your image on a complex background such as a gradient.
I found a suitable workaround. If I start with a png and get it to look right in the browser, I can grab a screen shot. Then I trim the outside of the image to make it transparent. Hence, the actual background is "hard coded" into the gif along the edges of where it becomes transparent so that true alpha blending is not necessary, but most of the image is still transparent so that the complex background shows through. Best of all, the gifs are a lot smaller than the original pngs.
I'm sure this trick is well known, but it was new to me. Oh the depths I go through to make IE users happy!