Even the best of programs (like NetNewsWire and Bloomberg) get crashy and it is amazingly common for apps to either freeze up, crash or (to my surprise for an OS X running machine) take down the whole phone entirely.
Often just restarting the app works fine (after a crash) or force quitting it if it has frozen (force quit by pressing and holding - for around 5 or so seconds - the menu button on the bottom of the iPhone).
Sometimes however, this doesn't work and often in the case of a new download you can erroneously (mistakenly, in error?) think that the app is, for a better word, crap. Sometimes it is, sometimes it isn't. A particularly handy to-do app called Zenbe seemed to crash on start-up, then I discovered lots of my apps were crashing. In this case, rebooting the phone often works (and in Zenbe's case it did when other things didn't). Reboot the phone by pressing and holding the power key (the one at the top) until you're prompted to turn it off.
Sometimes, if it's an app you've had on your iPhone for a while, it's best to make sure there are no updates available - most apps are updated regularly. This can be done either on the phone through the App Store or through iTunes by clicking in Applications in the library sidebar and going Check for Updates.
And, if the other reviews on the App Store indicate people are happily using things, the last thing I normally try is to delete the app off the iPhone (press and hold on the app in the menu window until it "jiggles" and click the cross) and off iTunes (select it in the Application list and hit backspace) then download it afresh from the App Store (it will be free if you've purchased it already).
So, to sum up, try:
- restarting the app;
- force quitting the app (if it has frozen);
- rebooting your phone;
- making sure the apps are up-to-date;
- deleting the app from both the phone and iTunes and re-downloading.
