1. Can you control who has access to your data?
Garmin won’t share your data without your consent.
2. Can you export your data?
You can download single tracklogs only, and there is no API (yet).
3. Can you close your account?
No obvious way to close an account.
4. Can the terms change without prior notice?
5. Can I trust them to keep my data secure?
Garmin Connect supports, but does not enforce using a secure connection (HTTPS).