Personally, I have found several benefits of a splash page. I consult on design for corporate websites and that is where I am coming from. So, some of this may not apply to you. I have found that a splash page can make a statement about the identity of the site in under five seconds and sometimes these things are important. when you move away from donationware to shareware, you may find this argument appealing. I have also used splash pages to prepare the rest of the site in the background as the user is identified (browser, OS, etc.) but stalled for a couple of seconds at the splash page. So, there are some benefits. Personally, I like cover pages, but I do realize it is a personal preference.
I am not a big fan of content on buttons changing on mouseover, especially when it changes size and rearranges all other buttons. Users tend to take a snapshot of the layout and store it in their, so to speak, cerebral RAM. I don't like messing with it and making them learn the page all over again.