Terms: http://www.withings.com/index/terms
Withings: http://www.withings.com/index/privacy
1. Can you control who has access to your data?
Withings won’t share your data without explicit permission. 
2. Can you export your data?
You can easily download all your data, and all data can be accessed through their API.
3. Can you close your account?
There is no obvious way to close an account, but “you have the right to […] eliminate data concerning you at any time”; you just need to “send your request by post”. 
4. Can the terms change without prior notice?
Withings “reserves the right to modify all or part of the Service’s Terms of Use without notice”.
5. Can I trust them to keep my data secure?
Withing’s website supports, but does not enforce using a secure connection (HTTPS).