This looks like a very cool, fast, easy way to get a store up in minutes. Like the iphone, this store needs no instructions. Very flexible, standard payment processor, shipping, etc. Doesn't seem to have complex config; eg, I didn't find a way to create required options or error messages to go with them. But for a simple store, my first impression is that this store is a dream.
For example, if you have a client with a few products, you could literally have this set up in existing site in about 5 minutes. If you want to customize the template, then it would take longer, but really, this thing is easy and FAST.
Regarding SEO, there is a mobile version of the store that can be loaded in mobile phones or ANY html browser, and this can be searched by search engines. There is an option to load this store on a subdomain on your site.
If money and speed are a bottom line for your client, this tool might be the answer. Of course, there is a down side: with this tool, your client probably won't need you
Also, sounds like there will be a paid version; maybe that will come with more bells and whistles.
Note: I was able to add this to a Joomla website in 5 minutes, and no formatting required.