Father's Day has been criticized by some fans for having a supposedly absurd premise which would effectively contradict the essence of the show itself. That's not the case. Instead, it's a pretty touching story that centers on Rose and allows the character to be a lot more than just another sidekick.
With the Editor defeated and the "boyfriend" gone, Rose asks the Doctor to take her back to the day her father Pete Tyler (Shaun Dingwall) died, since she never really got to know him and would like to see him one last time. The Time Lord complies, but warns her that if a time traveler alters his or her own life story, a time paradox will be created. Obviously, Rose doesn't pay attention, and so she saves Pete from being run over by a car. The consequence is that strange winged creatures appear and start wreaking havoc. Everyone (including a younger Jackie Tyler and an infant Rose) hide in a church, but that won't solve the problem: the only way to get things back to normal is to restore the original time-line, otherwise they will all die.
The main criticism aimed at the episode is that it's quite ridiculous for the Doctor to talk about paradoxes when he alters events on a daily basis (well, as daily as it gets for someone who constantly leaps through time and space). In reality, it's a widely accepted notion in science fiction literature, cinema and television (and it's fiction, not real science) that time travelers can do whatever they want, as long as they don't try to mess with their own history (case in point: the 2002 film version of H.G. Wells' The Time Machine states pretty clearly that the protagonist can't change his own past, no matter how hard he tries). This is also why the Doctor has never considered going back in time and saving the other Time Lords - he can't do it without possibly destroying the Universe.
That said, does Father's Day qualify as a good episode? Yes, because it does to Rose what the Dalek episode did to the Doctor: it gives the characters an opportunity to show the full extent of their internal tragedy, thus establishing them as real people (in the Doctor's case, as real as a 900-year old alien can get) and not just two goof-balls who use the TARDIS whenever they please. Dingwall, playing a role originally intended for Simon Pegg (who took the part of the Editor in The Long Game because of scheduling conflicts) adds plenty to the dramatic arc, and Camille Coduri gets to do a bit more as well, as opposed to the pure comic relief she was asked to provide in earlier episodes.
Any downsides? Well, there's the usual problem with some of the visual effects (the winged creatures look like video game monsters), but that's about it. Everything else - writing, directing, Eccleston, Piper et al - makes for another 45 minutes of above-average British sci- fi.
