I agree totally. However, in the last couple of years I've managed to get 2 mates with no previous 40k/BL experience into the HH series, and they are loving it, so don't be afraid to jump in if the mood takes you. As Chemos says, it might be better to read them in order, and not just concentrating on the ones dealing with the future chaos legions. Most of them cross over between legions anyway.
Of the 40k books, Ahriman:Exile is one of my favourites at the moment, with the Iron Warriors books by McNeill also worth a look. The Night Lords trilogy by ADB are widely thought of as some of the best novels BL have done in some years. There's no shortage of material for you to sink your teeth into.
Yeah, I got a few friends into reading 40K, and they started with the HH series, and they got pretty into it.
I still think that the HH series would be *best* if read with a decent knowledge of the 40K universe.
Luckily, I read the Blood Angels series before I read Fulgrim, so getting to see Fabius now compared to Fabius in the HH era was mind blowing as he is one of my favourite characters. That understanding of characters made me enjoy both books(Blood Angels book #4 and Fulgrim) much more than I think I normally would have, if I didn't know who Fabius was/is/becomes.
But ultimately, read whatever you want whenever you want lol.