When the App Store opened, I went nuts and downloaded a slew of apps. Two weeks later, I listed the 8 I decided to keep, including Major League Baseball's (MLB) At Bat [App Store link]. This week, At Bat has dropped in price from $4.99US to $2.99US.

It's on sale because it will "expire" when the current season ends (Go, Sox!), meaning you'll have to purchase a new version for 2009. Still, if you're a fan and into the post-season, I encourage you to buy it. It's my hands-down favorite iPhone application.

While using At Bat, I realized how an application's features and benefits affect its longevity. Specifically, the features attract your attention, while the benefits grab you for good.

Here's an example. At Bat provides nearly real-time statistics for ongoing games, including scoring, current batter and pitcher, stats, the count at the plate ... even video clips from a game that's in progress.

You can identify your favorite team to keep their score highlighted, and enjoy an integrated version of MLB Gameday, which provides pitch-by-pitch information and so much more. All of this is presented in an attractive, readable UI.

Those are the features.

The benefit is a feeling of connectedness, even participation, and that's why At Bat is a keeper. As a die-hard baseball fan, I want to watch every game. I know the stats, the players, the standings; who's playing well, who's on injured reserve and who's in a slump. It's tremendously fun to watch a baseball game on a summer evening and get lost in the drama of it all.

However, life interferes (can you say "toddlers"?) and I sometimes miss a game. Yet, those live stats and video clips alleviate the feeling of missing the game, and that's worth a lot more than $2.99 to me.

The same can be said of other applications. Twitterrific [App Store link] puts 1,000 friends in your pocket (leave your adolescent joke in the comments, please). At any time, I can share a funny picture, observation or idea with my buddies, and get a reply just as easily. Those are the features. The benefit is the comfort of friendship.

The benefit of FileMagnet [App Store link] is peace of mind. Yes, I've got that PDF for the meeting. Yes, I've got the photo I have to show my boss. Phew.

Others that I've eliminated went in the other direction - all feature. Cro-Mag Rally's [App Store link] accelerometer-based steering got boring after 10 minutes.

Magnetism Studios, the guys behind Insanely Great Tees, have just launched four iPhone apps, which they are dubbing "Magnet Apps."

Two of the apps are $4.99 and two are $2.99.

Here are the details:

Tile Sudoku ($4.99)

Sudoku is a popular puzzle game choice for iPhone devs, but Tile Sudoku has a neat little twist. Instead of selecting a number from a list, you drag numbered tiles to the correct space on the Sudoku grid. If you make a mistake, you can move the tile to the correct space or back to the tile bar. I haven't used any of the Sudoku iPhone apps yet (I have played some of the web-based versions), but this seems to make a lot of ergonomic sense.

You can buy it from the App Store here.

Mr. Shuffle ($2.99)

It's Mr. Potato Head on the iPhone! Mr. Shuffle lets you create funny faces out of cartoon parts. You can then add speech bubbles or captions to customize the images further. While all that is cool, the most fun part is probably the feature that lets you add elements to pictures you take of your friends from the iPhone camera.

You can buy it from the App Store here.

City Transit ($2.99)

Everytime I visit New York City, I dread having to carry around a transit map guide like a tourist. And even then, you never really know where the closest station is to your location or if there has been some sort of service change. City Transit aims to change all that. For $2.99, you get officially licensed MTA subway maps, a GPS-based station finder to find the nearest station (and it is integrated with Maps so you can get walking directions too) and service advisories from the MTA website, so that you know can quickly out about any line changes.

Magnetism has designed City Transit to have an expandable architecture and in the future you should be able to download maps for buses and commuter rail lines. This really looks like the perfect app for anyone who commutes to NYC.

You can buy it from the App Store here.

File Magnet ($4.99)

File Magnet, which right now will only work in conjunction with Macs running Leopard, is a way to transfer files to and from your Mac from your iPhone or iPod Touch over Wi-Fi and then view them. It supports PDF, DOC, TXT, RTF, RTFD, HTML, PNG, TIF, JPEG. GIF and iPhone compatible movies and audio files. Support for Excel and iWork files is coming soon. So instead of having to e-mail yourself PDF files, you can just load the File Magnet Uploader on your Mac and send it directly to your iPhone.

It does require Wi-Fi to transfer files back and forth, and is not compatible with Windows.

You can buy it from the App Store here.