If all you are looking for is a corporate instant messenger, office communications server is a complete waste of your time. I say this as someone only really looking for a corporate instant messenger who has the thing deployed. I am using about a tenth of a percent of the program's features in my environment. (My CTO <3s Microsoft.) If you need a corporate messenger and nothing more then get a jabber server and be done with it in 15 minutes flat. No fuss, no muss.
Now, if you need a completely integrated UNIFIED MESSENGING client, then I would put OCS up against anything out there. IBM and Cisco have competitive offerings…but OCS 2007 R2 goes toe-to-toe. What is unified messaging? Hell if I know. I am still figuring that out. So far as I can determine from the buzzword checkbox it is the ability for everyone in my entire company to attach a wire to each testicle and taser me on a whim.
This thing is stupidly powerful, and I am enough of a luddite for that to be enough reason to fear it. Assuming I weren’t using it from within a VM (which neuters about half of it’s abilities from the get-go,) then it’s kind of cool. I can have it control my IP PBX; it would do voicemail, call routing to a USB or Bluetooth headset, I could talk via phone or office communicator client to anyone in my organisation and I can set up a ridiculously complex auto-attendant feature on it. In truth, I could probably do all of that with a jabber server, Asterix and about 12 months to write a customer web interface for a series of complicated shell scripts…
…but this is push-button and frighteningly simple. (I say frighteningly simple because if my CTO ever figures out what the damned thing can actually do, I am terrified he’ll try to make me put it to use as more than an IM; a project that would be disastrous since we are 100% VDI.)
I can also right click on a contact and schedule a meeting with them, view their current busy/free status, when they will be free next (based on their calendar), send files, share my desktop (or a single application) with them, do things using SharePoint I only barely comprehend as well as customise lists, groups, teams and any other manner of personnel organisation you could conceive of all from within this one little application.
It ties into Office and into Windows. So I can be browsing a file in my documents folder, realise I need to send it to Alice, right click and send as e-mail, send to the contact, or open it up in a shared application session with Alice for mutual editing and review. Hell, it can schedule a meeting with Alice to schedule a conference call with Bob to do shared application session on the file if I only knew the right sequence of buttons to push.
Oh, and it has both Windows Mobile and Blackberry clients. The nerd in me looks at the technology involved and says “this is really, really cool.” The sysadmin in my looks at it and is TERRIFED. If middle-management types ever figured out what the thing could do…