I have tried tons of other jabber clients, and nothing beats Spark for the cleanliness and simplicity of the interface, and the organization of the company groups. The only client I've tried recently that had a similarly no-nonsense interface was the built-in Messages app for OSX Yosemite (but I'm not sure about its other features).
The only thing that continues to irk me about Spark are
1. various small bugs
2. lack of implementation of Message Carbons
3. lack of built-in whiteboard
4. lack of built-in video/voice chat
5. lack of support for server-side chat history (this also requires server support)
I was thinking that, my company would probably gladly pay $30 / month for someone to maintain Spark more actively. That doesn't sound like much, but what if we could get 10 more small businesses on board? Would $300 / month be enough to motivate someone to develop Spark more actively? Obviously it wouldn't be a full-time job, but some development would be better than none at all.