This has been discussed a few other times here, so you might want to do a search. I believe Dragon Naturally Speaking was brought up in the other two or three threads that I remember off-hand, so that might be a good search term.
I haven't used the latest version, but I have tried previous versions a few times and was always rather unsuccessful with it, even after hours of training. But I mumble and really don't speak clearly at all - hearing problems, so I can't hear what I really sound like, so, I dunno, I sound funny sometimes. Personally, I can type fast enough (80 wpm) that, combined with how slowly you have to speak and carefully you have to enunciate (or at least I would), it just wasn't worth it for me.
To increase the probability of it working well, it was suggested years ago to use a headphone/mic set that had the sound processor built in, rather than relying on a not-as-good sound board. I don't know if sound cards have improved enough since then to know if that's still an issue or not, but it's something to check out.
dadburnett, generally speaking, no. In order for the software to work at recognizing speech patterns, a person first has to go through training with the software. When I've done it in the past, it involved reading out sentences that the program gave me as it matched my voice patterns to what it knew the words should be. So, unless you could get the person doing the tape to train with the software first, you won't have much luck.