Yep! I have used them! I don't know that I'd say the price is "reasonable", it's $16-$18 a yard for quilting cotton. But designing your own fabric is a lot of fun and pretty awesome!
Here's Spoonflower's info page, listing their fabrics and pricing.
http://www.spoonflower.com/spoonflower_fabrics
And here's the blog post I did about the Spoonflower fabric I ordered:
http://www.seamstobeyouandme.com/wor...wer-christmas/
You can order swatches from them for a nominal price to see if you like it. Good luck!