I've been running it for the last day, and it's stable and fast.
Syncing to my Feedbin account works great; the Feedbin web UI is usable, but especially the last redesign leaves me somewhat annoyed, and it has very limited keybindings. NNW also has local OPML subscriptions, if you don't need to sync and don't mind waiting forever for it to fetch from every blog and deal with everyone's crazy broken RSS. I like Feedbin centralizing that nonsense, just replacing the UI has been a problem.
NNW has everything keyboard-driven, but I'm not enamored of some of its choices (Help, Keyboard Shortcuts). I'd prefer vi keys, and those are a dangerous habit with Brent's keybindings (l is mark all as read, next; k is mark all as read; I almost never want to do that, and want to hide those behind a warning). I'll either get used to them or see if I can rebind them from system settings. The code's on github, so worst case I can just fork it and hack my own keys in.
The reader's pretty nice. Stays in dark mode when I have that selected; I've seen a couple feeds insert their own background image/color which is obnoxious, but if that's what the feed contains, it should probably show it.
I miss in-app browser tabs. For webcomics (which rarely put full-size images in the feed) I often launch a bunch of them into their own tabs and then read them, which in Feedbin's web UI is (while more-comics? (middle-click title) (press 'space "next comic")) and in NNW is (while more-comics? (press 'b "open the current comic in browser") (press 'alt-tab "back") (press 'space "next comic")). Maybe it's possible to open a browser tab in the background?
A million times better than the years-late, rarely-updated Black Pixel release which had their own broken sync server.
Update: One feature I'm loving is drag-and-drop blogroll reorganization. Got a feed somewhere wrong? Drag. Drop. Synced. Fucking done. In Feedbin, you have to have the feed visible (unread or switched to "all" mode), the old way you'd hit Tags (folders) and just change the text in the dialog box, but they changed it to an Edit button which pops up a giant list of all your tags (folders) and you switch them on/off. Utterly awful if you use a lot of tags (folders) like I do. And yes, I'm annoyed perpetually by the "tags" concept, which doesn't exist in OPML, only folders.