Personally, they've mostly been very good. The product works, it was very easy to set up and it does everything out of the box, and it's made sales tax & accountancy almost completely disappear.
I have occasionally run into issues or bugs, and their API is a bit of a mess, but nothing show stopping and their team has been reasonably responsive and sorted everything out very reliably. That's noticeably got better & faster recently, I think they're beefed out their support team a lot in the last year or so.
If you're selling a new product as a substantial business, I think they're good but there are other options to look at too and there are tradeoffs (5% is high, you could probably do your own customer accountancy etc in house).
If you're a solo dev/small indie, or just getting started though I think it's a no-brainer. It's just so much quicker & easier than doing everything yourself.