Hi Scott! Thank you for requesting feedback.

What works well today with the SLA feature?

We rely on the SLA feature heavily in our support queues. We really like being able to arrange the view by the First Reply SLA for that group. It also works really well for processes that have Resolution SLAs and allow us to understand how often we are hitting the target there. The team really likes the heatmaps I'm able to create in Explore using our SLAs.
From an organizational perspective, we can tie SLA pretty neatly with satisfaction. This allows us to establish value for the reply times that we set and help us convince other departments of the need for more scrutiny of their own reply times.

What limitations do you hit while using it?

One of the main ones are tickets that have already met the SLA and fall to the back of the inbox. We have tried multiple ways of fixing this (using next reply SLAs, grouping by status because those come in as Open). I would like to be able to give some weighting to tickets that meet the SLA but should be concurrent in the view. These primarily come from one of two things, an agent is out of office and one of their tickets receives a reply or we have a ticket transferred from a team that is using our CRM, in which case the first reply was by the other team, and we're being asked to follow-up.
Another thing that we have run into is that we use the Zendesk app SLA Event Tracker. I would love to see that just incorporated into the interface in some way so folks can accurately see when a multi-SLA ticket is near breach on the non-focal SLA. Having a history of the SLA timer would also be useful.
We see a lot of confusion around what resolves a First Reply SLA with folks new to Zendesk. The first confusion is around private comments and SLA as Monica noted above. The second confusion is when we send out a proactive message and the user replies 3 weeks later, our first reply time gets tanked. We have created a workaround on this by tagging all of our proactive messages and then excluding them from First Reply SLAs, but it would be really nice if those tickets would count First Reply as the time between the user's response and ours, rather than from our originating message and our response. If we could reduce user confusion and get a more accurate SLA in these cases, it would be a win-win.