You either like him, or you don't. If you can get into his voice then you're guaranteed to love all his work. Those who do, really, really do like him. I personally prefer the live SM songs Simen sings to the album versions. It's all about opinions though, afterall.
And I wouldn't be talking about false vocals relying on programs to sound better, at least he has the guts to show himself live on stage, Garm has pretty much never done so, only a few occasions (they even needed to hire someone else to sing his stuff live with Arcturus, which btw the guy nailed), and the one Ulver bootleg I did see of him singing live was some horrid, shitty black metal voice that failed harder than a DVD Rewinder machine. Garm is a good studio musician, but he is the epitome of fake. Prove me wrong
