I'm doing Mod2 with BGS (material quality excellent as well known), but purchased the performance book on PadPilot out of interest and to have at least one subject readily available on-the-go on my iPad.
The quality seems high. Very readable text and nice diagrams. An in-depth comparison against material from BGS or OAA is beyond the scope of my use so far to be honest, but initial impression is a good one. They're not cheap at around £55 per book - but I guess that compares with the OAA books retailing at a similar price, for example.
If you have the notes from your groundschool you don't need these of course unless you fancy reading another text, or if you want the notes on an iPad (as I did).
I think the other providers might want to consider the iPad/Kindle/etc going forward...
Hope helps.