I have used several shopping platforms and I consider Magento to be far superior to other shopping platforms that I have used – although, at first I was pretty critical of it.
My initial criticisms of Magento (particularly versions before 1.4) were around its performance issues. However, they have improved this significantly and are getting better although like anything, if you want Magento to be fast and scalable you need to have some knowledge of PHP and Web Server configuration so that you can easily optimize Magento’s caching to your particular store(s).
Having said that, some "quick wins" I found to improve performance and scalability of Magento were:
Reduce HTTP Requests - Optimising your theme, cropping your images etc
Use a CDN
Enable Block Caching where it makes sense - have a look at http://www.magentocommerce.com/wiki/5_-_modules_and_developm... for more details
Instead of using mod_php use FastCGI to Run PHP – if you have to use mod_php turn KeepAlive Off
Delve into the Configuration settings & disable modules which aren’t essential to your store - System -> Configuration -> Advanced -> Advanced
Likewise, some other big/notable players using Magento include:
Warby Parker - http://www.warbyparker.com/
Overstock Deals - http://www.overstockdeals.com/
Gap/Blue Navy/Banana Republic/PiperLime/Athleta – http://www.gap.com