Interesting article on Machen on The Guardian blog which gives a reasonable assessment of Machen's significance and the power of his works has attracted a lot of coverage on the web.
As one comment pointed out it is perhaps not fair to say Machen is entirely forgotten - amongst genre fans his name is still fairly well known, as Machen appears in countless anthologies of the best horror stories. Of course amongst educated Forteans Machen is remembered because of the Angels of Mons. Certainly amongst the general public he is unknown now though.
Interestingly Machen's reputation is actually higher abroad in some respects than in the UK. Mentions of Machen in the Spanish press equal or exceed mentions in Britain. The anthologies of Machen by Borges still have a big draw. In all it is a good summary of Machen for a general reader.