I am a MASSIVE fan of Desperate Housewives, but this was the poorest series out of the four they have done so far. I watched every episode and purchased the DVD and still re-watch them, but I wouldn't say it was a particularly strong series.
I'd say the first 8 episodes were particularly strong, not forgetting the sensational episode 'Bang!'. Despite it being in the midst of a weak series, it's by far one of the best ever episodes and gives me goosebumps at the end every time. But yes, the first third of the series looked particularly promising but from about Episode 10 - 20 it just went downhill for me.
I thought the seasons mystery was lacking the particular oomph the others had. Season 1 was just a huge puzzle, Season 2 finished with the unexpected twist.. but I found this mystery surrounding Orson/Alma/Gloria to be slightly dull in comparison and predictable - It could only have been Gloria or Alma, as it being Orson would have been too obvious. Maybe the weakness has something to do with the fact they decided not to make Orson a villian at the last minute... He was originally supposed to be in league with the woman he visited at the psychiatric ward at the end of Season 2, setting out to destroy Bree.
After the mystery was solved prematurely due to Marcia Cross's pregnancy, they lacked quality I thought. The storyline with Rick wasn't too bad in the end, I felt sorry for Lynette when she had to fire him - some great acting from Felicty. Mrs McClusky's storyline was pretty odd with her husband in the freezer. I would have preferred if that wasn't in there but I guess there had to be a filler storyline with Bree missing. The relationship between Carlos and Edie just didn't seem right to start with either, but it wasn't so bad after a few episodes.
I think the last three episodes, 21 - 23, really picked up again, with the revelation of Lynette's lymphoma, Susan's hilarious trip to the woods and Edie's desperation at the season finale. Bree's return was a good surprise for the end of Season 3 and lead nicely into Season 4, which is just fantastic.