The Slitheen have always been my favourite kind of villian in the first season. I'm not sure why because they are not really in it apart from being 'inside' human shells but this episode and it's twin (the one after) are really well done!
The whole coming back a year late was an interesting take on the whole time travel thing and sometimes you do it get it wrong. I think in the last two episodes you kind of forget that Rose has a life back on Earth, even though she does call her Mother from the End of The World space station thing, you don't really realise what she left behind, then in this you see that in fact the world did continue without her and it really missed her!
This leaves of at a very interesting cliff hanger!