So, to run down some key points:
- Can totally change the look of a site.
- Can hide SharePoint components that you don't want to use.
- Will allow you to alter the layout of the page, in addition to changing the colors and images used in the site.
- Will not affect _layouts pages. You will need to use a workaround.
- Can re-skin the layout of a site to use different images and colors.
- Can only hide SharePoint components that can be controlled through hiding it in the CSS (display: none set on a class/ID).
- Affects _layouts pages.
- Can be used as an alternate CSS file application method, similar to alternate CSS setting in MOSS sites.
- Are installed on the web server unless you customize the file.