Sorry, this won't be a rave...
As I understand it, the new block is supposed to appear in BlockBuilder as it would do live on the built page. But what you see in BB is not what you are going to get.
In the best case, what you see is the look and feel [and even then, more feel than real look] for the appearance in your current browser, with your current screen resolution and window size.
This should be especially true with responsive templates, as they are designed to adjust to whatever device they are displayed on.
Even with my fixed Content Column width, this is what I see in BlockBuilder in Firefox on Windows [Clear is set to Both, image width to 248px, original image is 480px wide, my Content Column is 520px]:
This is what I get previewing the page:
This is how the page will look like on an iPhone 6 Plus in portrait:
...and in landscape:
To each her own, but I'll stick to the old "pure" Text and Image blocks, as long as they remain available -- their possible removal is hinted at in the help.
If this removal is to come, I'd strongly consider retaining at least the Text block, since with the new Text/Image, you need to click twice before you can start typing if you don't need an image.