Ooh, a topic that I love, pencil drawing, especially realist pencil drawing.
I'm also a pencil artist and as others have mentioned, time, patience and attention to detail is key in order to render a photo-realistic drawing.
I noticed that somebody mentioned they prefer the looser style of drawing from a viewing point of view and I guess it's just down to personal preference. Personally I love looking at drawings that are, at first glance, mistaken for a photograph but on closer inspection they are drawings. I love the process involved in drawing a photo-realistic drawing and have often taken 80+ hours on an A4 sized drawing. I spend hours sometimes on an inch square portion of a drawing, making sure I capture every last detail and often more detail than would be visible in a standard photograph.
Photo-realism comes in for a lot of stick from people who don't understand why anyone would 'copy' from a photo or reference of some kind just to end up with something that looks the same as the original photo and this annoys me a little. It's criticized without taking into account the amount of work that is needed to end up with such a drawing. People say things like "why didn't you just photocopy the thing in the first place?". Well to answer that, there's no skill in pressing a button on a photocopier and where's the enjoyment in that anyway? Also, that misses the point completely. Like I said, for me it's the process involved and the ability to add even more detail to a drawing than is visible in a photograph. A photograph is often limited due to how the lense only captures certain things in focus etc,. However, when drawing, I often take those out of focus areas or areas which are too dark to show any detail in a photo, and recreate the detail that I know must be in that area of darkness or out of focus, so in my opinion I improve on the original.
Anyway, this is a great thread and one that I hope to add too from time to time.
If anyone's interested, some names to look out for and who specialise in realistic photo drawings are Armin Mersmann, J D Hilberry, Linda Huber, Paul Lung and of course 'Mightyfine' AKA Peter Williams, oh and maybe myself

. I know a couple of them have been mentioned in this thread but all of them are worth checking out.
Below is an example of one of my drawings if you're interested. This drawing took close to 150 hours to complete.