Same here, use PI from the first day it showed up on iOS (as I loved it on WinMobile).
Back then, PI couldn't use the iOS calendars because of the iOS limitations and my calendar resides on an exchange server, so I had to dump it for calendaring but used it for ToDo. No real issues since then.
I also sync with toodledo iOS app without any problems.
The only Programm which REALLY messed up anything was 2Do. Althoug greatly designed and seems to work well with any other sync-option, it totally screwed up my toodledo and always deleted 1 or 2 tasks with each sync from iPhone to iPad and back. But that's another program.
I can recommend anyone to try PI in the latest version. It's really great.
If you have an older device (3Gs or older) you could be disappointed about the speed when using the built-in-calendars of iOS, but that is because it always has to sync it with the internal calendar, Apple doesn't allow direct write access. With iPhone 4S or iPad 2 it's really fast, although it could be seemless if Apple... you know the deal ;-)