They are quite good with nice general tutorials. Took a look at their openvpn ones when i was still at another provider. You can also sign up with leaving little data and they have been around for a while. I also never saw a mullvad ad, i think they live by (imho earned) word of mouth.

Thats not to say that they are perfect, your experience may vary with your use case, but in general i havent seen anything to complain about. Not something you can say about many of the other providers.