This was an outstanding episode - a perfect mix of action and wit, tension and humour. The opening sequence with the blowfish and the old lady was just perfect - especially the "Bloody Torchwood!" comment.
James Marsters is great as Captain John Hart, and right from the start manages to be equal parts despicable and weirdly likable, though perhaps personality-wise a little too reminiscent of a certain other character he's well known for playing, who had that same style of friendly amorality. The interaction between him and Jack is highly entertaining, especially in the scene where they first meet.
Only two things annoyed me a bit: First, Jack's persistent refusal to open up at all to the other characters about his past or his reasons for doing anything, which is understandable to a point, but starting to get to a level where you really want to smack him. He makes such a point of how he values the Torchwood team and how they're so important to them, but he can't get it together to trust them enough to say anything whatsoever about his past? Despite the fact that they deal with weirder stuff than that on a daily basis? It's really kind of aggravating...
Second, the odd and sort of random way that his relationship with Ianto seems to be developing. Don't get me wrong, I'm not objecting in the slightest to the two of them getting involved. I actually get really annoyed with people who complain about the gay/bi elements of Torchwood - in case they haven't figured it out yet, that's one of the show's major themes and one of the reasons a lot of us watch it! Anyone who can't handle that should probably stick to watching Doctor Who.
But I just think the development of this particular relationship hasn't been handled as well as it could be. First it seemed like Ianto hated Jack (in Cyberwoman), and then just a few episodes later (in They Keep Killing Suzie), you have that odd little innuendo about the stopwatch which seemed to just come out of nowhere, at a time when you'd still expect Ianto to be grieving for Lisa. And now in this one you have Jack kissing Ianto when he first reappears, which suggests they already have some level of romantic involvement, but then the exchange while searching the office sounds like they don't yet and Jack's still testing the waters. It's all kind of confusing and hard to follow, and you don't really get any sense of a relationship naturally evolving the way you did with Gwen and Owen in the first season.
However, those are only small complaints about an otherwise great show and episode.