With its new Lion operating system for Mac computers, Apple has unleashed its most complete rethinking in a decade of what it means to use a computer.

Apple calls this the post-PC era, a reflection of the trend away from boxy computers and toward mobile devices such as smartphones and tablet computers, highlighted by the enormous success of the iPad.

Apple sold a staggering 9.25 million iPads last quarter -- a 183 percent increase over last year.

Mac OS X Lion (10.7) is a major step forward and well worth its $30 price in the Mac App Store. Lion was downloaded more than 1 million times in just the first day, Apple said recently.

Here are five pluses -- reasons to take the Lion plunge; and two minuses -- reasons to sit this one out.

Plus: full-screen apps

Apple has rewritten its apps to be able to run in full-screen mode, which looks especially great on its laptop computers.

This is a decided nod to the iPad, which runs all of its apps full-screen. Apps like iPhoto, Mail and iCal look incredible as they stretch from edge to edge.

Mission Control view allows for a quick look at which apps are running in full-screen mode so users can switch between them.

Plus: multi-touch gestures

It takes some getting used to, but Apple has reversed the direction of trackpad scrolling in Mac OS X Lion. It sounds crazy, but it makes the Mac experience the same as the iPad and iPhone.

Instead of scrolling down to read further on a Web page, users pull the page up by dragging their fingers upward on the trackpad. Those who can't get used to this can change the preference, but it quickly becomes intuitive.

Other handy gestures include the ability to quickly swipe left and right between full-screen apps that are running side-by-side.

Plus: fast Web browsing

Safari, Apple's Web browser, has been rewritten for Lion to embrace the full-screen functionality and is blazingly fast -- so fast that it encouraged me to make the switch from Google Chrome.

The new browser also includes a handy feature called Reading List. This allows users to flag pages such as news articles in a sidebar, creating a list of links to read later.

One improvement is needed: The list should sync across all of a user's computers and devices.

Plus: launchpad

Another feature lifted in part from the iPad, Launchpad allows for quick access to a computer's applications, which can then be opened in one click as they spread across the screen in a grid.

Applications can also be organized in folders based on their purpose or frequency of use.

Plus: auto save

In applications written for Lion, such as Apple's word processor Pages, documents will be saved automatically as they are written. There is no need here to select File > Save -- each keystroke is recorded and saved.

The writer can also take a look back at all the saved versions of the document and revert if desired.

Minus: no physical media

The only way to upgrade to Lion is to download the new software through the Mac App Store. This might be hard for some who have a slow Internet connection or who want a hard copy of a computer's operating system.

Apple says customers can visit an Apple retail store to download the software or wait until the company sells Lion on a USB drive for $69 soon.

Minus: not for old or slow computers

Apple recommends Lion for computers with 2GB of memory, although it will run snappier on machines with at least 4GB. It also requires a fairly recent computer with a high-powered processor. Users must already have Snow Leopard (10.6) to upgrade to Lion.

On Mac computers bought in 2007 or earlier, Lion might be incompatible or slow the computer enough to make the upgrade a bad idea.