The first season really didn't do anything for me, I didn't really like the stories nor the characters. So I gave the second season a miss the first time it aired. I only recently started to watch it after reading that it did improve, which so far it has.
Now, not being blessed with all that much patience (and also having read spoilers about season two and watching Doctor Who which also gave a lot away) I watched season three already.
It helps that it's only five episodes, and it's also amongst the best episodes I've seen of Torchwood and Doctor Who combined.
It was a really dark story, which didn't have all that many slow moments in it. Just a few silly scenes though and why there had to be all those tentacles and goo I don't know.
This was how I supposed Torchwood would be at the start, also a bit how it was presented. It makes me hope that there'll be a season four, with the same level of quality.
Several of the previous episodes and also Doctor Who episodes seem to suffer from the fast pace it has to have as everything is resolved in one episode most of the time, sometimes there's a two parter but those are rare. Sure, there's always a sort of basic theme throughout a season, but that's vague and at times ridiculous. At least this time they were willing to spread it out over five episodes, allowing for a more detailed and deeper story. If only they did that more often.
In short, when this airs and you like Doctor Who or Torchwood already, definitely watch it!