Amusingly, we (BugHerd) have exactly this sort of system in place. Almost universally our users prefer to get individual messages. For a lot of these notification emails, just the subject line is enough for most people to know whether an action is required or not. Grouping the notifications means you _must_ open to the email to see what's going on = more work. Possibly the biggest complaint is that if you're collabor…
Interesting. I think ultimately user control is what you want here. For example, Trello offers a choice between Instant, Hourly, and Never. For now Meldium uses a heuristic to batch so if only one event occurs then you still get the subject line brevity that you mention.
My similar implementation offers these emailing options (every, digest or none). The nice thing about this approach is you can just add logic in the post_commit handler to route the notifications appropriately (instant email, digest queue or /dev/null appropriately based on user preferences)