Conversations (mentioned in a different sub-thread) is a nice example of a niche instant messenger focusing on good UX. It is not only standards-compliant, but the author is actively contributing XMPP extensions, moving forward the state of XMPP art.

I don't know much about niche messengers which are not based on XMPP, but most XMPP clients happen to be niche projects themselves, maintained by one or two persons in their spare time.