The iPad and iPhone both offer Push notifications, but not every application or web service has been upgraded to use them. Boxcar acts as an intermediary, letting you know as soon as anything happens, whether it's on Twitter or RSS, Facebook, Foursquare, Reddit, Google Buzz or more. And it does it very well.
The real problem isn't with the Boxcar app itself, but with Apple's handling of Push notifications. They're incredibly disruptive, taking you away from whatever you were doing and completely taking over, to the point that getting more of them is likely to be the last thing you want.
It's one thing to take out your iPhone and see at a glance that you have a new message, but the same thing popping up in the middle of a movie or game is just annoying.
What makes it even more of a problem is that while Boxcar will alert you to updates from multiple services, its badge can only show you how many alerts you have waiting, not whether they're important. Both of these things render it less than useful, although it does an excellent job if you don't mind the downsides of Push notification.
It is, of course, only a stopgap measure, as more and more applications take the job over directly.
For the iPhone, Boxcar is still a handy tool. On the iPad, however, you're probably best off avoiding it. Whatever you do though, don't ask for Push notifications on something like #wikileaks, or just fighting your way to the screen that lets you turn it off will become a nightmarish odyssey in its own right.
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