Fiora is strong all game and is very fun to play with. I especially like her bursts and speed. There wasn't even a minute, that I regretted buying her. Also it's quite easy to play with her.

Also it's my first time creating a build/guide, so I would be happy if you write any suggestions in comments.

You can also use armor penetration runes instead of critical chance.

Pros:

Very good burst damage
Very useful in team fights
Good movement speed after Burst of Speed

Cons:

If you get stunned, you're out
Low on health(but they can't always target you!)

Don't rush using Q second time, it can help you escape over enemy minions. I suggest you to jump on enemy with Q, then use E and W, then if you think you need R and at last second Q(but all of your enemies will be dead by that time).

I do not suggest you to use Q when farming minions(because it has quite long cooldown). You should use E more often when killing minions.

I failed with her only 1 time(till now). So my scores were(i like to rush into battle and turret dive):

12/2/4
9/11/3
5/0/0
30/8/11
11/14/9(i rushed to0 much and felt like a god too much too)
8/11/3
14/3/0
11/9/7

You can watch my plays, if you have LOL replay named:

Just being Fiora(wand3r)
Oh you, Fiora(wand3r)
Fiora(wand3r)