I just installed the Macintosh App Store program and found it very stable and compelling. Last Wednesday (5 Jan 2011), the Mac App Store app became available after installing updates to the latest version of Snow Leopard and noticing the new Mac Store App Icon in my dock. (Snow Leopard IS required to install the Mac App Store on your Macintosh). I immediately downloaded a free utility Caffeine, that lets you temporarily suspend screen savers. Worked like a charm.
To test whether I could download one paid app to multiple Macs (I have 3 older Macs) with no extra charges, I downloaded 50% off Angry Birds at $4.95 to my ancient MacBook Pro. I moved to my second machine (an iMac) and started the Mac App Store download of Angry Birds. It's a little disconcerting because you have to purchase the same app, enter your password, THEN, it tells you you already have the app, and that the download is free. So don't make a mistake or you may make an unwanted purchase. I couldn't resist a Mac Web Site editor (Rapidweaver) that was 50% off at $39 and another $4.99 app Courier, a utility to simplify upload content to multiple Web 2.0 sites like Flicker, Facebook, Youtube, Mobileme and more.
The search for apps is similar to that experienced on the iPhone and iPad so I was using the Mac App Store very quickly and effectively. Apple claims over 1 million downloads the first day, so it looks like a hit AND it's a lot of fun! The 7.4 mb Mac Store App is called "App Store.app" in your Application folder.