You can use it with status.net (like identi.ca) and with twitter.
It has many features like attachments, bookmarks and filters . It supports tags, groups and public timelines, favor/disfavor notices, subscribe/unsubscribe users.
You can add multiple accounts also with merged timeline.
You're invited also to join the mustard's user group
You can report Bugs and request Feature using LaunchPad
If you are interested to help us translating mustard in your language mail me or ping me on identi.ca!!