I've used them before - one thing to mention is that Spark isn't really the "client" - it's a Java implementation of the Jabber/XMPP client protocol - so it's more like the API that you would need to develop a client.
If you want is a full-featured ready-made IM client, you may want to look for an existing open-source IM client that supports the Jabber/XMPP protocol (there are tons out there), run OpenFire as your corporate IM server, and block outside IM servers from your network
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