My collie bitch is three in April and she is really lovely. It took me a while to bond with her and gain confidence in my ability to encourage her to respond to me, but I love her to pieces. DP and I have worked with her and she, as a rule, is a very well behaved dog, if a little fizzy 
We do have two major problems with her though. Not anything that would make us consider giving her up, but they are having a considerable impact on our lives.
The first one is linked to the second one, so bear with me! She adores the cat and the cat tolerates the dog, but she is heartbroken when the cat goes out. For example, when I am in, the window is always open so the cat can come and go as she pleases. I don't really want to have to change this because of the dog, because I don't feel it is fair for the cat. When the cat hops out the window, the dog hates it and squeaks A Lot. I call her in to the room I'm in to try and distract her, but generally she doesn't have it and sprints back to the room that the cat disappeared from. She does settle down after about five minutes, but the cat comes and goes quite a lot...
The second one is that there is a period every bloody evening when the cat jumps down from her bed on top of the book case and tries to come for a cuddle. The dog goes absolutely berserk. I can't tell if it is jealousy or not - part of me thinks it isn't because I can try and cuddle her and she doesn't care - she is fixated on the cat. The cat will try and move about the house and the dog will chase her.
I am prone to believing that the dog, because of her breed, is trying to herd the cat, but I want her to stop it. The thing is though, she gets so fixated on the cat that she shakes. She will, after being asked several times, respond to a 'watch' command where she has to look me in the eye, but she will just return to looking at the cat.
The chasing of the cat is getting to the point where DP won't come in to the living room in the evenings because they both drive him crazy. This is obviously upsetting because we end up spending our evenings apart.
I did take my dog to puppy classes over a year ago and they suggested squirting her with water every time she does it, but it didn't work and I didn't enjoy doing it. I also think it was sort of counter productive, as I want my dog to actively relax instead of becoming even more tense.
Sorry it is so long, but please could someone suggest something? I am reading up about the 'stop' command to make her stop squeaking, but I am really at a loss as to how to stop her fixation with the cat 