Pip is definitely a reliable narrator; he comes across as being very genuine and honest, even in aspects of the story that could be seen as personally damaging, he tells it faithfully. If anything, Pip's youthfulness could be seen as a liability to the reliability of his narrative, because children often view events in such extreme perspectives, like they like something or hate it, find it horrifying or delightful. Dickens keeps Pip's account pretty focused, which adds to the overall likeability of his character.
I think Pip is one of the most reliable of first person narrators, although events are coloured by his personal perspective, and reflection often obscures some of the detail even in a real account. That said, I think these weaknesses humanize Pip as a character more, and is therefore as reliable as any real person would be, reflecting on events and characters as diverse and wide-ranging as Pip does.
First person narrators must always be viewed with at least a little suspicion, since we all see events through our own unique perspective. When I read Great Expectations I thought Pip was reliable because he grew as a character over the course of the story. His point was to show how and what he learned about life, so I think he did a lot of soul-searching as a character. He was able to look at his own weaknesses and fears and report them to his reader.
I think Pip is a very reliable narrator! I believe it is only right that Pip narrate his own story. Stories from a personal perspective allow the reader to actually live inside the character and feel/see things from that perspective. I enjoy personal narrative stories much more than the ones in which the story jumps back and forth between one or more characters. Personal narrative stories leave much more to the imagination of the reader as he/she guesses or surmises the reactions, feeling, thoughts, and actions of the other characters in the story by their facial features, posture, and tone of voice.