I guess this depends on what you consider sounding great. If you listen to the voices most of it may sound ok, but then you get the robot effect where you have pauses and improper pronunciation.
Plus they don't divulge anything about the licensing requirements or royalties that may be imposed when using this product in a project that you sell. I know this because I had once did a lot of research into using text to speech for a commercial product. A lot of my research found that a lot times these companies were not strait forward about their licensing requirements. In other words the product that the voice was produced in may not have cost anything but the backed voiceToSpeech editor had royalties attached.
Here is an example. My company sells a product using a different vendor as our database engine provider. Our product costs x dollars but then we have to pay royalties to the database provider for use of their product. For each copy of the software we sell our database vendor makes z dollars. The same thing applies to most text to speech voice generators on the web.
Again, I would use caution. Most of these text to speech editors have one backed and as we know works derived from someones work is considered copyright infringement. You have to do your homework. What may appear to be free or cheap may in fact may cost you in the long run. You also need to be aware that with the internet, some of these companies offering these products may not be operating in the US which means they may be operating in a country where the US cannot enforce our copyright laws.
Now I'll step down off my soap box. It's just been my previous experience, you get what you pay for.