Prior to having a PDA, I never thought having a voice recorder would be anything incredibly beneficial. However, once I got a Palm Zire 72 with the voice recorder, I found myself using it all the time (especially when driving, with the one button access). This continued through my Treo 680, but when I got the iPhone 3G, I was dissappointed to see that it didn't have a voice recorder option.
Of course, soon enough there were many options available in the App Store. The one I choose to use is simply called "Recorder".
In my opinion, the hallmark of a good voice recorder is simplicity. I don't want to have huge numbers of options, many diffrerent buttons to push, etc. I simply want to be able to record a quick memo or thought, before I forget it. In this function, Recorder excels. When you start the program, there is a large red button at the bottom of the screen. To record, simply tap the button. To stop recording, you can tap it again. Nice and easy.
Now, if you want to, Recorder lets you have some more options. You can change the name of a "memo" so you can easilly identify it. The memo indicate the day it was recorded (or the time, if it is still the same day) as well as the length of the recording. It sorts them chronologically (newest is listed first). To listen to a memo, you simply tap on the name, and a play/pause button appears to the left. In addition, you can use the progress meter at the bottom to replay any part of the memo (especially handy if you are trying to transcribe something).
To delete a recording, you can use the old "side swipe" method, or use the mutliple delete option under the "edit" button. You also have the option of emailing a memo to yourself in AIFF format, so you can keep a copy of something or use it outside of the iPhone.
The one thing I wish that Recorder did was give you the option of recording phone calls. But as far as I know, there aren't any voice recorders that can do that with the iPhone, so it may be a hardware limitation.
I also wish that the iPhone has some additional hardware buttons, or at least enabled you to customize the one button more than you can do now (for example, a push and hold of the home buttom, or more options to choose for the double tap of the home button). You can always make Recorder one of your four permanent virtual icons, but usually people use those buttons for programs that are access more readily. So as it stands, you need to find the Recorder program, then tap it, then tap to record - a little more involved that the dedicated one touch recording I had with the Treo.
But those problems which are more of a problem with the iPhone itself than the program. Recorder does exactly what you ask of it, which is all you can ask for in an application. This program costs $0.99. There are some free ones, including some with more features (such as transcribing your memos) but this one does everything I need, is easy to use, and has a simple, elegant interface.
I am giving it 5 out of 5 stars, with no cons (other than the hardware limitations).