I shared some of the previous reviewers' reservations about a spin off - SG-1 was a bit of a legend after all. No need to worry because Atlantis started strongly and has gone from strength to strength. The characterisations are strong, the plots have the wide arcs we've come to expect from this franchise and the individual episodes also stand in their own right. Yes, some of the episodes are stronger than others - show me a TV series that doesn't have variations. Also, while there may seem to be initial 'similarities' with characters from SG-1, give it a couple of episodes and these disappear, to be replaced with individuals who gel together as a team extremely well and offer something distinctly different to the O'Neill/Carter/Jackson/Teal'c quartet - a difference that is very refreshing.
The quality of the visual effects are outstanding for a TV series (much, much better than SG-1) and the pilot in particular is superb. Other highlights are the mid-season two-parter The Storm/The Eye and the several other episodes that deal with the Genii. The Wraith are a much scarier nemesis than the Goauld could ever be and the whole series has a much more apocalyptic feel about it, a sense of jeopardy - which makes for much more involving TV.
Stand-out characters are Joe Flanigan (Major Sheppard) and David Hewlett(Dr Rodney McKay) although it's a bit unfair to single these two out as the cast is an ensemble and all give fine performances. On a personal level I 'like' the Atlantis characters far more than I 'liked' the SG-1 ones - which is a testament to both the quality of the acting and the writing as well.
Overall, an excellent series that can be watched by anyone who loves Sci-Fi as well as people who just enjoy an exciting story with cool baddies and some very, very funny one-liners. Slight warning here that some very young children may find the Wraith a bit much.