No experience, but just looking at the website, I don't like them.
a) They pretend Amazon and Kindle are different things.
b) Since a) already makes them look dishonest, the way the "testimony" under the "Sell" box almost-quotes the "Sell" box makes me suspicious they made it up themselves.
c) You have to pay $150
for them to publish on 4 platforms plus their own, which they pretend are 5 by pretending Amazon and Kindle are different things.
d) I get the impression they are mostly interested in upselling services, including charging you $49 if you'd like to change the price of the book. Seems like you even have to buy your own ebook copy if you want to see if formatting turned out OK...
e) They take a 20% cut (example calculations for ebooks at the bottom here)
Smashwords AFAICT takes a smaller cut, and certainly distributes to more ebook retailers (including Kobo, which I see as important because they don't limit sales to the USA).
I guess if fastpencil actually does good work with the editing services and such, that might
be worth it - it would save you work looking for an editor and formatter yourself - particularly if you are confident about selling a lot of books. But I have no idea about that.