So I bought Fiora first week out, something I usually don't do because they often get nerfed. But I played against her and saw how broken she is (verse most other solo tops) and I had to buy her to help me get my second account to level 30. She has very good single target damage and is very good duelist with her having the best attack speed steroid in the game. I find her very good at snowballing as well, if you get a early kill from a gank or just from outplaying someone that you can easily boss your lane around and zone them. Some things that counter her are other champs that can out harass her. I played against a Wukong one game and it was brutal, she doesn't deal with that very good at all.
I like building her with a Wrigles, Phage/Frozen Mallet, Merc treds, Bloodthrister. and usually by then game is over of close to over. If you need to be more tanky you could add Atmogs to this and I could see it working. I like the build that I first said because it gives you good armor and health. Anyways I am in no means a master of her but I can see me playing a lot of her, she is VERY fun and her ulti animation is awesome to say the least.