In my opinion after playing GWE two days ago it has a busy and great timetable however they aren't challenging or worthwhile. As I can just put my hst on max throttle until reaching Reading or not being able to drive a complete service due to where the services end etc.
I wish they would remove the turbo scream from the GWE HST sounds though, they always ignore my support ticket but it doesn't sound good at all.
The HST journey is far too short sadly, hopefully in the future we might get a new route maybe to Bristol or Swindon at least. The class 166 services are the only ones I play. I don't think the timetable is as busy as it is in real life or was then, even taking out the Heathrow Express you never see Paddington station empty during the day.