In Exchange O365 is there a way to let users release their own emails in quarantine without Exchange admin intervention?

As above but we've bought Mimecast to do this https://www.mimecast.com/resources/o365/

Once enable is it for all users or we still can restrict access?

it's for all users. each user that gets spam quarantined will get it.

What is the pros and cons of activating that option?

since you know what the quarantine is its your choice to allow user access to the reports so users can release spam to their mailbox. you need to determine pros and cons for that. whatever your company allows. I personally like the feature.

Depends on your users. The ability for users to release messages from quarantine can be a real time saver for both admins and users. To be honest, it's better to give them access to this function because they are bound to get at least some spam to their inbox anyway (so they need basic spam awareness), and when they actually do get an important non-spam message into the quarantine, they are happier if they don't have to contact admin just to release the message.
On the other hand, if you have users who go click happy whenever they see a link in an email, blocking the quarantine feature might minimize the damage.

Thanks