I really like this episode. I love the atmosphere of suspicion and fear it creates - something that's not very common for Trek. Also, I like that there's no TNG-esque hand-wringing about what they have to do - there's a Changling on board and they have to kill him, end of story.
I also like that TPTB started to really accept that the show was becoming a serial at this point. Ira Steven Behr himself has stated that it was with this episode that they began to plan far in advance for future episodes, and it shows. Not only are they laying the groundwork which will dominate the Federation-Dominion Cold War, they're also setting up side-plots (like Eddington possibly being a traitor).