Hush, like any company or individual, is legally bound to respond to court-issued subpoenas. However, because not even Hush can access the encryption keys of individual users, in the case of a subpoena Hush would only be able to provide the encrypted (coded) version of the transmitted email.
When someone informed BugTraq of a security exploit they found in the leaked NT source code, they used Hushmail. If an uber-hacker thinks it's good ...
Hushmail was probably inspired, in part, by a desire to protect people from the DMCA. These are folks who fear the US is turning into a police state. (Hah, hah, wherever do they get that silly idea. Not from the Patriot Act, CAPSS II, the DMCA, Patriot II ...)
It has other likely users too. Watch for Patriot III to ban it.