We've been using the preview for a few weeks, and while it's nice to have an in-built option for lightweight planning, Planner still pretty rough around the edges.

It's missing features one might expect from something tied into 365 (of course subjective):

1. Weak outlook integration (e.g., can't send a card as a task, email, or calendar invite)

2. Weak sharepoint integration (e.g., can't browse for links to attachments). It supports onedrive, but just for the logged in user.

3. Limited (read: one) reporting or charting options. Maybe Gantts are a bridge too far, but there are many useful visualizations beyond who has outstanding tasks.

4. Can't move cards to new boards (just new buckets)

5. Can't clone/copy cards

And it's just plain buggy:

1. Many links aren't actually links, but rather click handlers on divs, breaking common UI like "right-click, open in new tab".

2. Icons and previews have busted caching behavior, whereby "correct" icons show for a while (sometimes seconds) before being replaced with old versions

3. Cards will duplicate or disappear while editing, usually fixed with a refresh

4. and cetera.

There's potential for new features leveraging the 365 ecosystem, but its as yet unrealized, and it has a long way to go before matching its most obvious competitors in polish (UI and code quality, not necessarily style).