What informs part of my warnings is that I've known some people who jumped into The Silmarillion and were much put off by it. They were expecting a gripping fantasy narrative and what they got was something that seemed translated out of the Elder Eddas with a little bit of Holinshed thrown in.
TS remains near and dear to my heart but I try not to make the mistake of thinking that everyone will love it like I do. Or at least that they know what they're getting into when they read it.
They're more likely to appraoch it in the right frame of mind if they read and fall in love with LOTR first.
Those that don't care for LOTR - who find it long and lugubrious and overwritten - will be put off even more by The Silmarillion.
I think the canon/non-canon issue is really a secondary one anyway.