The improvements are minor, but this wasn't intended to be a dramatic overhaul of the Gmail platform. It's definitely convenient to be able to check your calendar, notes, and tasks all within your Gmail tab — and that means two fewer tabs that you would have needed to open.
The inbox does look noticeably different, but barely. It is nice to be able to see exactly what attachments are in your inbox, but I found that it adds more clutter and chose to use the 'comfortable' inbox view.
So the biggest reason to switch is probably the plugin integration — other than that you're not missing much. And if you switch over and decide you don't like the change, you can easily switch back to the old Gmail.