I was planning on using RoR for an upcoming e-commerce site but keep reading comments that give the impression that this framework is not yet fit for this type of a commercial application.
What is the state of it? May it be used for large scale online stores etc? Is compatibility and the frequently changing framework an issue to worry about?
yes, rails can do it quite well. check out active merchant site, as well as shopify, which is a rails site specifically for ecommerce. as far as changes to the framework, i've found them pretty easy to keep up with on our applications.
It depends on what features you want in your system.
Chris mentioned some tools for payment processing and order / cart management.
If you want something for a 'large scale online store' then I'm not sure there is anything yet. Specifically when I worked with ATG Dynamo Commerce Server, there was a massive amount of effort invested in their pricing, discount and promotion elements.
Late answer... but I feel I should share some of the latest stuff available.
Is compatibility and the frequently changing framework an issue to worry about?
No. But it sounds like you are a beginner with Rails -- anyone starting a big e-commerce project should consider sticking with a framework they are already comfortable with.
If you definitely want a Rails app and your site is primarily an e-commerce shopping site, I recommend you look at Spree (http://spreecommerce.com/).
If your site is primarily a web application with a small purchasing component, consider using ActiveMerchant directly to hack together your own simple shopping page.
If you are making a primarily e-commerce shopping site, you would do well to at least consider non-Rails solutions. There are so many options for standalone web stores, shopping carts linked to CMS apps like WordPress and Drupal, third-party services for a fee (e.g., Shopify for shops, Chargify for subscription services) and many of them can be customized with minimal coding.
Rails is up to the task, but if you aren't already a "Rails guy" there's no particular reason to choose it for a new e-commerce app.
Agree with other's posts about activemerchant. That combined with someone like Braintree as the gateway (who are programmer oriented) make it pretty simple to do.