The Start Screen is like the start menu except full screen and more customizable and manageable than the menu was. As it's more visual, you end up finding things quicker. Although, there are some that dispute this fact.
My opinion is that the Start Screen is perfectly acceptable in a touch screen environment, but useless on my desktop or PC. I don't have a need or desire to have a "full screen" launching point for my apps. A couple of desktop icons, a couple of pinned taskbar entries and that's all that i need. I don't like being moved away from my desktop (and seeing emails, instant messages, etc). when I launch the full screen Start Screen. I don't like the "live" nature of the tiles. They are distracting and a waste of bandwidth to me. Not to mention, they get confusing when your People app is showing a facebook pic, your photo app is also showing a pic...and you cannot easily tell which tile is which. I don't like having to clean up the Start Screen when the program installer litters it full of tiles that I will hardly ever use. And I don't like the fact that I don't have any classic start button options without going with a third party whose integration may or may not be stable, reliable, or secure.
I don't really use any of the "new" apps because I'm annoyed to no end that they are full screen or only able to use 130px of the screen of the entire screen. I generally don't like getting to application settings via the Charms bar....i much prefer an in-app File, Edit, View menu etc.
I'm a Windows fan...but not a fan of Windows 8.