I came late and backwards to Torchwood; I watched the first episode before I'd ever watched any of the relaunch Doctor Who episodes. I quite liked it but mainly I wanted to watch what they'd done to Doctor Who. The revamped Who was so great that it kept me busy until I saw all of it, then I went back to watch Torchwood.
It's a great idea for a series but unfortunately, the writing remains very inconsistent. To my mind, the show is at its weakest when there's a lot of running around after scaly-skinned beasties, and at its best when it stops to think a bit. The series finale, for example, is a disaster - overblown, humourless and silly, it's redeemed only by some subtle acting, mostly from John Barrowman and Eve Myles. However, only a few episodes earlier there was the wonderful 'Out of Time' episode, a quiet and rather sad tale which began at a point where most shows would have ended the story and which dared to be about people rather than about monsters. Elsewhere, the grim farce of 'Countrycide' is a witty homage to slasher movies, just as 'Combat' is an unimaginative and unconvincing rip-off of 'Fight Club'. But there's a refreshingly 21st century openness about sexuality. Captain Jack will flirt with anything that exhibits sentience, and when he finally gets to have a big old snog (with a WW2 airman as chiselled as himself) it's a moment of old-school romance. Elsewhere, both Eve Myles and Naoko Mori's characters find themselves curiously aroused at different times by persons of the same sex. It's a long way from the chaste (but increasingly lovelorn) world of the Doctor.
The acting is as mixed as the writing. John Barrowman is effortlessly charming, but perhaps because his Doctor Who episodes were better written and better script-edited, he's a more convincing hero in Doctor Who than he is in this. Here, the writing sometimes fails him, so that when he's supposed to come across as a leader of men, he seems more like a louche playboy who is a bit exasperated by the whole situation. It's hard to tell whether Burn Gorman is a bad actor, or Owen Harper is a badly-written character, but I suspect it's a bit of both; in any case, the actor relies way too much on narrowing his eyes and trying to look well 'ard. Gareth David-Lloyd is quietly efficient as the support guy Ianto until he has to carry a whole episode - then he goes way over the top. For my money, the best male actor in the show is actually Kai Owen as Gwen's long-suffering but basically chipper boyfriend Rhys. He manages to suggest both ordinariness and a basic sense of his own worth and dignity, a tricky thing to pull off.
The women are on the whole better than the men. Eve Myles is both gorgeous and a really fine actor; Naoko Mori squeezes every drop out of the sometimes exposition-heavy role of Toshiko; it's a bit of a shame that Indira Varma was only used in two episodes, as Suzie was in some ways the most interesting character in the show, just as Varma is in some ways the best actress. (Her performance as the resurrected Suzie is truly chilling.)
My wife and I watched Torchwood over the course of a summer holiday, and we could never be sure in advance if an episode was going to be completely brilliant and original or merely an efficient but generic bit of forgettable sci-fi nonsense. Buy the box set at your peril; it's not as good as Doctor Who, but Doctor Who is (mostly) so very, very good these days that Torchwood still manages to be remarkably fine for a spin-off. Still, the show is frustratingly inconsistent. I do think they could have come up with a better series finale than a CGI demon stomping around Cardiff zapping people.
One last thing - kudos to the creators for setting this thing in Cardiff at all, and making the place seem fascinating, haunted and sexy.