Powerstore is very easily customizable
I can tell you that PS is totally customizable. Here is what I did with it. (www.SoleusRunning.com)
There may be a better way, but it worked great for me.
What I did first was created a template and made it look like I wanted for the site and then I took the Powerstore pages and incorporated them into my new pages. Very simple to do. Basically copied the code and that's it.
I wanted to keep all the pages for Powerstore in original state.
For each page, I created a seperate page for the store and copied the code from the original page from Powerstore. For example, cart.php is from PS, I created a page (with my template) and called it soleus-cart.php. Same functionality, but with my look.
Hope this helps.
PS. Plus one really great benefit is that the folks at Webassist are 100% awesome on providing support. The forums are really top notch.
PSS. Also, if you do end up making new pages like I did and calling them something different than what the original files are called you will need to make sure they are changed inside the code. For example, if a reference was made to cart.php I had to change it to reference/call soleus-cart.php. There's only a couple of places that this needs to be done. (checkout.php, cart.php confirm.php)