In reply to NorthernGrit:
You can pay attention to a film or podcast when doing over/under's at 120% ftp?
This is what I never got with TR. Nearly every serious cyclist I know who uses it tries to convert the sessions (albeit good sessions and plans) onto another system to keep themselves entertained. I tried it myself, but if I'm doing 3 hours low intensity it's outdoors for me.
I've done a lot on the sufferfest. It sort of depends what you want to get out of your training. If you want to use your indoor sessions as compressed 45-90min HIT training, and be entertained and kept going through the pain then the sufferfest has it nailed.
If you want to substitute the indoor trainer for all of your riding - low intensity, steady state and maybe even racing - then Zwift is a lot better. As you go further on in the Sufferfest lengths the sessions just get stupid (great, thanks, 300tss of hit intervals for 3 hrs, kill me).
Also it depends on how you structure your training plan and where it's coming from. The plans on SF are more rudimentary, whereas the TR and Zwift are generally better thought out and more progressive. For me - I write my own ATP on Trainingpeaks in conjunction with my coach and so the SF just slots into the "indoor HIT" bracket. The rest is done in group sessions, on the road, or at this time of year - mainly - in the gym.