ActiveRecord, MySQL, and nested transactions — what's the behavior?

Question

Rails uses savepoints to achieve nested transactions with MySQL, and to my understanding, the semantics of this are identical to actual nested transactions, in terms of atomic data changes.

1. Is this true?
2. What about calling "save" at arbitrary times within the code? The transaction still stays open until the end of the block, right? Are there any differences in behavior when using nested transactions/savepoints?
3. Anything else to be aware of?
4. [intentionally inciting flame-war] Should I switch to PostgresSQL?