• Yes, it's Windows only..  For now.  We are looking at other platforms and other areas as we move forward.
• The Notes Browser Plug-in is a subset of the Notes Client, and main selling point is that it opens your content inside a browser.  So if you are using iNotes and this, you are staying within the context of the browser, rather than switching to a full-blown notes client.
• If you are not using iNotes , then a Notes Client is needed in your case as you need to still access mail using the client (Plug-in does not support mail)
• What the browser plug-in installs is much much smaller set of files as opposed to the full-client, all that is needed to run your applications.  
• Can you tell us what you are using for mail and if you are planning to use iNotes?  That is the main use case.

• With this plug-in, we still must install software in PC.
• Now, the size of the disks is in term of hundreds of Gb, so 0.3 or 0.4 Gb of difference is not so relevant.
• Our main problem is to run Notes Aplications in Tablets, because to develop or to migrate Notes Apps to xPages or Web, it take a lots of time and money.
• In term of costs, is it free or cheaper than Notes licences?

I realize I'm a late on responding to this, but to answer your question, my company uses Exchange for email.  We did use Lotus Notes for many years, but then migrated for consistency throughout our organization.  We still use Lotus Notes for a large number of workflow applications and we still generate email alerts on the Domino server, but the email just gets forwarded to each user's Exchange account.