The objective of the RSS Awareness Day is to get as many people as possible talking about RSS and its benefits on May 1st. Bloggers and blog readers are already aware of the RSS format, but if enough of them talk about it perhaps the mainstream media will cover it as well, and the general public will get exposed to it.
If people start reading about RSS and how it can help them everywhere, I am sure that some will get curious at least to try it out. Over the time the usage of the format should pick up, and all the Internet users (including content publishers) gain from it.
I use Google Reader to keep up with blog reading. It makes life easier and faster (I don't have to browse through "favorites" to read and try to keep up with folks who don't post often.)
The down side is that they may not catch my reading (it doesn't show up on sitemeter) and I don't catch folks who use reader to read my blog. So numbers may be different than indicated.
I'm sure there's a way to find out how many people subscribe through RSS - I just don't know what it is.