I only discovered Alias last year. For four years it had slipped under my radar.
It was through Lost and the whole J.J. Abrams connection that I decided to give this show a shot, and I was not to be disappointed. In fact, I ended up loving it so much that Lost was put into the shadow for a good few months.
Seasons 1 & 2 are the most polished pieces of TV work I've ever had the fortune to experience. Drama, intensity, emotion, mystery, surprises... everything you could possibly desire. There really isn't a dull moment in the entirety of the first 48 episodes.
While it might be true to say that the show lost its way slightly in Seasons 3 & 4 - perhaps too many standalone episodes and a few rehashed ideas - there's no question that it was still very enjoyable and still much better than most shows.
And though Season 5 suffered slightly due to being shorter than most fans (and no doubt the writers) would've liked, Alias was right back on form for it's final run as it built towards a fitting climax.
To summarise, I can't speak highly enough of this show. It has become my favourite TV show of all time. For high tempo action fused together with intriguing mystery, Alias is as good as it gets.
Buy this set and own every episode of the most under-rated show in history.