iDialer is designed to replace your number pad with a more modern looking, iPhone-style skin. Thanks to the large buttons and clear design of the interface, the application is very finger-friendly, improving the speed in which you can tap phone numbers in.
iDialer includes a few basic telephone functions, such as contacts, a start call button, and call history. These are all easily accessible from a menu underneath the number pad. You can even skin this menu, although this involves a lot of faffing around with the settings.ini file, so it's not recommended for inexperienced users.
The main problem I have with iDialer is that it doesn't really add any new functionality to Windows Mobile. Sure, you get a nice iPhone-type keypad, but it's really just a shell whose appearance doesn't extend anywhere else, such as your Contacts, for instance.
If you're desperate to make your Pocket PC look like an iPhone then iDialer might be worth a look, but don't expect any revolutionary changes to the way you make calls.