It is bad indeed. I've just started using it in anger as my team has all adopted it and as a result I'm angry.
The UI is inconsistent and dysfunctional. It breaks paradigms that have existed since Windows 95. e.g. why does the checkbox on the left of your list of tasks mark the task as complete, rather than acting as a multiple select tag like it has since forever? Why can I not drag and drop a task card to a pinned plan? I can double click a plan in "My Plans" and it takes me to that plan, but if I double click a task in "My Tasks", it just edits the name - I have to right click and do "open details" - basic stupid inconsistency.
In our pilot there are a dozen teams, and they've all created a plan called "Tasks" in one or more of their channels. I have tasks in several of those, so when I look at "My Tasks" I see a load of tasks assigned to me on different plans, in different teams, but I'm literally not given a clue that these are not all tasks on the same plan.
And yes, updates (e.g. creating a new plan) are laggy, like I should be grateful to sit there for minutes waiting for my plan to appear in a list so I can move tasks onto it.
Also I don't see how this is possible, but I can create plans, but I can't move any tasks to them (even tasks in another plan in the same team) and I can't delete the plans I created. This may just be a permissions problem on my tenant, but the move operation gives me an utterly useless cryptic error message so I really can't tell without getting a sysadmin involved.
I worry about the future if this is a sign of the competency of Microsoft's developers.