The problem I have with HEX is that it doesn't really take you to central London. Paddington is one of the more inconveniently located stations in London and anything which involves the 5min walk to platform 16 and a journey east on the Circle / Hammersmith and city line is always painful and has the potential to wipe out any time savings.

It would be a lot better if HEX went in to Victoria or Waterloo which would make it more viable competition with the Picadilly line which serves the tourist locations a lot cheaper (and about 10-20mins slower) than HEX.