As I'm sure you're aware you should be getting a large group of new users as IWantSandy shuts down (yes yes I know there is now an open source version in the works but thats not the topic!)
I exported all appointments into an ICal and imported into Toodledo with no issues. The file is not over 1MB as specified on the upload page and checking the file it appears to be a standard ICal file.
The problem is that not all of my appointments have migrated and initial investigation suggests that Toodledo does not like the RRULE:FREQ=WEEKLY tag used by Sandy. I assume this was used by Sandy to note it was a weekly occuring task but I have not checked if it is valid ICal syntax.
While I will continue to investigate on my own and keep the thread updated: Has anyone else experienced this and already found a workaround?