What are the implications of implementing Database.Stateful?

Daniel Ballinger: No, batches do not ever run simultaneously. You are correct, however, that serialization is the culprit here.

grigriforce: what's your batch size? If you're doing a million records, and your batch size is 1, then you will serialize/deserialize your state 1M times. Even with a small serialized object, that's gonna hurt.