This is not a bad episode of Star Trek. However I do understand why so many people don't care for it. The first 3/4 's of the show are enjoyable I think, it's fun, exciting, it keeps you on the edge of your seat for the most part, and it really does a good job of coming off as a real Star Trek episode.
It's the ending that really left me with a bitter taste in my mouth so to speak. I thought it was very poorly written and an extremely unecessary plot development. Also, the explanation of what is happening to Paris is a little forced I feel. Most people have serious issues with the whole 'evolution' concept of this episode, it's not believeable, it's impossible etc etc. Well, that's Star Trek, get over it. But that's not what bothered me, it just didn't feel like a natural conclusion to this story, like I said, it felt forced.
All in all it's not a bad episode, hardly 'the worst ever'. But it really does go over the top with the ending, and the writing in some scenes, writing that will really have you reeling in disgust. But check it out anyway, you might like it.. Ya never know.