I have SeaMonkey installed on a couple of my PC laptops. It is really just the continuation of the Mozilla Suite.
I believe that development of the full Mozilla suite recently ended at 1.7.x so that they could focus their efforts on Firefox and Thunderbird. A sidegroup decided to continue the full suite concept and thus SeaMonkey was born.
I can't really tell the difference between the latest SeaMonkey release and previous versions of Mozilla. The main reason to get SeaMonkey is that you'll get the latest security updates.
I'm a huge fan of the full Mozilla/SeaMonkey suite concept. The browser is excellent, the email software is full featured, and I've also made use of the web page builder component. Several years ago the big knock against the full suite concept was the high memory requirements and the large size of the application. These days those worries are laughable since computers have so much more memory and huge hard drives. In fact, the full Mozilla/SeaMonkey suite takes up less drive space and uses less memory than installing both Firefox and Thunderbird.
I love the ability to use "skins" since the basic look is just plain ugly without one. I'm using the Orbit Retro or Orbit 3+1 themes http://themes.mozdev.org/
It works the same way in OSX and on a PC. It's got all the features most people will need and it's got great compatibility. For those of you who like me are still using OS9 get the Wamcom version of Mozilla http://wamcom.org/
I haven't even bothered with any other browser or email app for several years now since all the components work so well and are nicely integrated.
In terms of speed the browser runs as fast as anything else out there. The app does take longer to launch than some others, but once it's started it's speedy.