OK, as a sibling comment points out, it's been a harsh judgment (although I make it humorously).
In general terms, I think it's subpar in terms of usability, at least, when talking about "Google standards". Going down to details:
1. the web interface uses a sort of "dark pattern": it enforces delayed updates, to the point of ignoring user requests for refresh; when the user clicks on the refresh button, s/he'll see the spinner in action, but when it stops, there's no guarantee that the updates have been sent - in fact, attempting to close the browser at the point frequently shows a browser alert, then the spinner will spin again for the "real" update. It's a matter of seconds actually, but it gives the impression that the program is "lying" to the user.
2. the automatic recognition is extremely annoying when a note includes numbers and symbols that don't represent phone numbers/email adresses - Keep will forcefully recognize them as such, so that when one clicks on an auto-recognized string, a dialog will pop up, forcing the user to click Cancel, then hope that the landing character is the right one, otherwise, when clicking on another character, the popup will show again.
3. the reminders workflow is bizarre; it assumes that the main use case is the user setting alarms at four semi-fixed (fixed but customizable) times of the day. this doesn't really make sense to me, and adds one extra stage in the setup
4. at some point it seemed they introduced a clever reminders workflow - when the user would add one, the default would be around three hours from creation (with the exception of the night). this is not stable though - sometimes, the three hours interval is not respected.
5. I recently hit a bug where, when I select it from the Android task manager, the window closes.
Point #2 is definitely the worst for me; it really bites me on notes that include numbers and are frequently updated (eg. a gym schedule); I think this is a legitimate use case.
Point #1 may be a bug, although it's in a core functionality, which makes me think that it's intended.
News flash! I've just found a bug. There are four semi-fixed times of the day; but in the settings, one can only customize three.
Practically speaking, I think there isn't (enough) dogfooding in its design. More abstractly, I really doubt that it's up to standards with a company that is supposed to have the best IT professionals in the world.