Has anyone tried using planter boxes for flowers?  

Well since you bought the soil and seeds already, I guess you just pour in the soil and plant the seeds as it says on the package..If you hadn't bought the seeds already, I would suggest getting marigolds or pansies that are already started and transplant them..That way you can have an instant flowers already..

I live in an apartment with a porch and a railing around the porch so I purchased a planter box made of redwood. I am unsure what it is lined with.
I put potting soil in it and then planted some fuscha geraniums and they have grown quite well, very hardy and abundant in blooms. I have in the past also had sweet alyssum which is a small lacey white flower.

I think that you can grow a lot of flowers in the planters. I normally have started with small plants and not seeds as I wanted instant gratification. I think you should be fine using the planter.

Good luck to you in your planting, etc.

All you have to do is follow the directions on the seed packages. You can plant something that will grow over the sides like Ipomoea, Campsis or Dolichos. If you use a trailing plant with something different for color on the top it will probably be engulfed by the vine. But it is all up to you and your preference.

I recommend covering the baskets to keep the soil moist before the seeds germinate. Any clear plastic will work.

Make sure you water your coconut fiber pots often. They will have a tendency to dry out. Especially during the summer. That is the only bad thing about them.

Good Luck