I have used Teams before, but it's not smooth. Had a lot of UX cramps.
We've been using Teams for about 18 months since switching from Slack due to its prohibitive cost across our whole org[1]. Whilst Teams is far from perfect, it's got considerably better during that time.
[1] We'd reached the point where the 10000 message limit on the free plan meant our history was down to a few days, which meant that important messages were getting lost whilst they were still needed.
Why not pay for it? It's $6.67 per person, per month. Doesn't seem very expensive for a company.
For us, we need single sign-on, so Slack's £9.75/user/mo. For 200 users that's nearly £2000/month, or £24000/year.
We're now nearly 300 people. That's about £35000/year. And it only gets worse as the company grows.
That's more - I mean, way more - than it costs us to license SQL Server (amortised annually on a 3 year upgrade cycle). It's more than the licenses for several of our other business critical systems and services combined.
In terms of priorities then, we're about to embark on a series of infrastructure improvements and I'd much rather use that £35,000 there, where for us it can add more value.
The problem with Slack is it sounds cheap but the cost creeps up to quite substantial levels for even relatively small organisations. It then needs to be weighed against other costs, and the value it provides versus the value they provide.
Whereas on the other hand we're already paying for Office 365, which everybody in the company uses heavily every working day of the year, and Teams is bundled with it, and it does everything we need, so why not use it?