We use Microsoft Communicator at work and it integrates nicely with Microsoft Outlook. We've been using it for quite a while and people are used to it and it has been dependable. I really have nothing bad to say about it other than the cost, and whether it is worth it or not is really a business decision.
Still, I have examined a couple other platforms recently that are worth consideration. First I tried a chat plugin that integrates with MediaWiki. MediaWiki is a really nice plug and go solution for online documentation. There is an extension called "Chat" which integrates PhpFreeChat into the pages. It took a little tweaking and only worked with the 1.3 version for me instead of the 1.2 version recommended. The problem I encountered is that people are uncomfortable with chat room style systems and even though it does have private message capability built in, people have little incentive to use it if they are used to an Instant Messenger like Communicator and, this is key, people are loathe to expose their spelling, grammar and momentary lapses of memory to a large group. If you're looking for a single group chat system, I can recommend it, but it wasn't for us.
I am trialing OpenFire as an alternative. It seems to be pretty much on par with Communicator, but it is of course free. It can run on a Windows or Linux stack and integrates nicely with an Active Directory platform. This I can highly recommend. If your company is looking for a chat system, and you don't like the idea of paying for Communicator, then OpenFire is a good choice and pretty easy to set up.