"Is an iPhone an appropriate digital recording device?" asks James.
I would say only in a pinch, James - if there's absolutely nothing else available at the moment. Generally speaking, however, I would say no.
I assume you are asking with regard to capturing electronic voice phenomena (EVP). When it is a paranormal investigator's intention to record EVP, she or he should use a recorder of the highest possible quality. A dedicated digital voice recorder, set to the HQ (high quality) setting is usually fine for this purpose. And a good quality cassette tape recorder with high-quality tapes work well, too.
The problem with using the microphones and recording apps on smartphones - whether iPhone or Android - is that they are not really meant to record very high quality. And high quality is what we want when we are seeking EVP.
Grade A EVP - voices that are clear and unambiguous - are relatively rare. Most often they are faint, somewhat hard to hear, or open to interpretation. So having a recording of low quality only adds to these problems: Was that really a voice? Or was it a dog barking outside or a chair being moved across the floor in the next room? A recording of high quality might be able to make that distinction. A smartphone might not.
As paranormal investigators, we want to gather the very best evidence we can, and that goes for audio recordings, still pictures, and video. So we should always strive to use the best equipment we can afford. Good, solid paranormal evidence - good enough to give skeptics pause - is hard enough to come by, so let's not give them ammunition with the reason or excuse that our equipment was deficient.