Dark Angel has all the things that a great sci-fi TV show should have: a solid cast, decent production values, good scripts, and a detailed, consistent back story (or "mythology" for all you Joseph Campbell fans) that ties it all together. The first season was great and the second season upped the ante considerably, making for many tense, compelling story arcs. Indeed, the overarching plot of the second season--the conflict between fugitive genetically-engineered super-soldiers and the members of a secretive, selective breeding cult that's thousands of years old--made for some interesting, freaky viewing. Dark Angel also has the distinction of being the only show that I know of that is regularly referred to as "post-apocalyptic" and yet it has nothing to do with a global nuclear war (or a worldwide plague, or the Book of Revelation, or climate change, or alien invasion, or zombies), a quirk that the show's writers explored in a wide variety of intriguing ways. Unfortunately, all Dark Angel lacked was good ratings, which (like many sci-fi shows both before and after it) led to its cancellation after the second season, long before it could fully explore many of the ideas it presented.
Another thing to note (being the media production trivia geek that I am) is how many of the cast and crew of Dark Angel either came from or went on to other hit TV series; it's a veritable who's-who of hit TV trivia. Sure, Fox frequently dropped the name of James "King of the World" Cameron, Dark Angel's co-creator and co-producer, as a way to promote interest in the show; Cameron also directed the last episode of the series. (Even though Cameron is largely thought of as a filmmaker, particularly of big-budget, special-effects-heavy movies such as Titanic and Avatar, I really think that Dark Angel ranks as one of his best works.) Dark Angel also launched the career of then-unknown Jessica Alba. But take a look at Dark Angel's other alumni: Rene Echevarria, Ira Steven Behr, and Nana Visitor were previously involved in Star Trek: Deep Space Nine. Michael Weatherly would go on to NCIS, Jensen Ackles would go on to Supernatural, and Alimi Ballard would go on to Numb3rs. Furthermore, if you're paying attention, you'll notice Aaron Douglas, Alessandro Juliani, Kandyse McClure, Rekha Sharma, and Rick Worthy--all future cast members of the re-imagined Battlestar Galactica series.
So, if you don't mind being frustrated by two seasons of action-packed buildup and no conclusive payoff, with plenty of great stories and hit TV trivia name-checking in between, give Dark Angel a chance--you'll be impressed.