Well, you have to remember that I'm a Stargate SG-1 nut and I have all of SG series on DVD....but assuming that you know that:
Stargate Atlantis is fantastic - think of it like a more streamlined and finer worked SG-1. The team behind it had a lot of practice with SG-1 by that time! I love the characters and you get some cameos (both big and little) from the old team too.
Now in the timelines of the series, the humans in SG-1 in our galaxy are getting a bit too powerful, so the premise of SG Atlantis is really to reset that dynamic with a new enemy, being cut off from earth - so the Atlantis colony are on their own - and a whole lot of mystery. There's some great set piece story arcs, which I won't spoil, and some wonderful episodes. 'Sunday' always makes me cry - goddamn you SG.
I can't say if you might find it dated, I don't know. If you can watch some series 4-7 SG-1 and you still like that, this'll be your cup of tea too.
I will say that series 5 of Stargate: Atlantis is a bit flabby and tired and they try and tie everything up, and so I find it a bit hurried, but that's the problem with a lot of TV shows, that are going from series to series not knowing if they will get funded. I'll let them get away with that for the four series before.
Have to say though, my favourite is Stargate: Universe. So annoyed they could only get two series of that