If you only have a few bookmarks, it is a really nice addon (I will not repeat the testimonials - agreed with most of them). On the other hand, I have 15000 in 1000 folders, and a new tab with this addon can load for 4 to 10 seconds.
Folder search (not doable natively) is a big plus for large bookmark collections. In my case, however, search speed is also a little lacking.
One feature that I really miss is linking to individual bookmark folders. It seems to be pretty rare in "bookmark-new-tab" extensions. If folders had individual local addresses, you could:
1) bookmark them for cross-linking between folders;
2) load them in the sidebar as menus (native Firefox function for bookmarks);
*) all sorts of other things: toolbar placement, virtual folders / junction points, tiles... You could also tune the new tab to show this or that specific folder, not the whole tree, leaving that for the toolbar button.
Except for search, all of that is already possible with a much less popular addon with an almost identical title: