SpeechCloud's VoiceDialer [Free, App Store] was the first iPhone application to try to offer speech dialing on the iPhone. VoiceDialer takes advantage of the iPhone's always-on internet connection to record your voice and send it to SpeechCloud's servers to perform the actual recognition. Once recognized, the application pulls up the contact's name and allows you to select which number to dial. Some of the criticism of the application is that it requires too much manual interaction (tapping on buttons) to actually dial a number, and slow response time due to the transferring of data across wireless networks.
Next up was Makayama's VoiceDial [$14.99, App Store], which has drawn a fair amount of App Store commenter criticism for it's high price. VoiceDial takes a different approach by avoiding actual speech recognition and instead performing audio comparison. VoiceDial requires you to actually record your own voice for each contact which can then later be used to match your voice command. A video demo of the product is offered by its publisher. If you are willing to pay the $15 and willing to record yourself saying your contacts, MercuryNews claims the product "works as advertised" and "had no problems recognizing the contact I wanted to call, even when it was similar to other names I'd recorded."
Now comes HRL Technologies' VoiceThis Dialer [App Store], a $9.99 application that actually tries to perform speech recognition within the iPhone itself. No wireless connection required. Instead, the application runs within the iPhone. VoiceThis Dialer promises to offer completely hands free activity with the ability to dial contacts and even quit the application with your voice.
Unfortunately, during my brief testing, the application performed poorly with voice recognition. In fact, I had to cut testing short due to it persistently dialing the wrong number. There is a confirmation mode that can be turned on, also by a voice command, but ironically, I couldn't get it to recognize it. With no audio or voice prompts at all, I found myself looking at the phone continuously to see if the audio level was reading or if the proper number had been chosen. This visual attention negated any benefit from the addition of a voice dialer to my phone.