This plug-in works just fine (unlike many others I have tried). But I have serious issues with obfuscating email addresses by using images.
First of all, I had to spend 20 minutes revamping the images for "@" and "." to adjust the color, font and baseline alignment so they matched the links in my template. Luckily all my links are the same color, and don't have a different hover-color, because these images inserted by plug-in are used for all email addresses. Not exactly the most flexible solution.
Also, the images are one fixed size--according to the CSS, 10px tall. They stay this size regardless of what font-size your links may be, so they may easily look too big or too small. The images themselves are actually 2-color aliased GIFs images that are quite a bit bigger than 10px, so they're being scaled down, probably with the goal of anti-aliased edges. Unfortunately, most browsers don't do a good job of this anti-aliasing on their own. The following CSS code somewhat fixes this and sizes the images relative to the surrounding text.
Other problems include: the images also don't print well. You can't copy-and-paste the addresses correctly. Any styles applied to images as a whole (in my template, I have a border and box-shadow automatically applied to all images in posts) have to be removed for these images, and you won't get some of the styles applied to the surrounding text (like text-shadow).
All in all, a workable solution, but hardly ideal.
More info on anti-aliasing: