At first glance this seems like an episode for O'Neill's character to develop, but it's actually more about the characters around him, and also sheds light on his backstory for viewers who hadn't seen the film. It's a great showcase, too, for Richard Dean Anderson's acting talent. He plays both O'Neill and an alien impersonating him, learning about his past, and has some really emotional scenes.
This is the first planet-of-the-week that I really enjoyed a lot, due to the totally alien nature of the crystal life, and the heart-wrenching recaps of Jack's pre-stargate life. Sam learns for the first time about O'Neill's ex-wife and the fate of his son, and Teal'c takes an opportunity to watch the TV news and learn about the "strange place" that is the planet Earth. Meanwhile, the alien-O'Neill helps Sarah O'Neill (Jack's ex-wife) move past Jack and the loss of their son Charlie.
It's a shame these crystal aliens never returned as it was refreshing to see extraterrestrial life that wasn't a dude in prosthetics. It all helps to flesh out the world of stargate though, and even if they were just a tool for character development, it was a pretty cool way to do it. The early seasons had a few episodes like this - just to show cool aspects of the universe that were out there, without necessarily bringing back the characters, races, or stories. Some of them that may have been better received or easier to integrate into future episodes did return - the Nox showed up a few times, as did the Tollans, and the Tok'ra became a fairly major character group. Other times, such as this, or the episode with the Re'tu, races were introduced and wrapped up within the episode, never to be seen again. I suppose it was a consequence of the show still 'finding its feet' - an excuse I seem to have made several times already.