It is or can be a pain to setup
we have had several bugs
1) stopped being able to use outlook out of office assistant
2) Cannot synchronise with address book (red ! on the icon in system tray)
Can be a real pain in the ass
But all the messaging side works
I know OCS 2007 pretty well - it's probably the main thing I work on now. I was involved with the original airlift for all the betas out in Seattle.
Happy to answer any questions you may have +++
2 above is quite common - AddressBook is distributed via IIS (HTTPS) on your front-end servers. Most people do the OCS bit but then forget to do the IIS bit (I.e. assign relevant certificates etc.) also there's a couple of other bits that could cause this.
Overall it can be quite a complex product, depending on what functionality you want. Simple Instant Messaging and Presence - well that's easy really.
Add in external access via an edge and it gets complex - there's all kinds of certificate dependencies, never mind firewall and NAT issues for media.
Same with telephony. Integration to stuff like Avaya or Cisco for remote call control (I.e. Communicator controls your phone) is relatively simple, start doing Enterprise Voice (I.e. Softphone) then it gets more complex.
Loads of stuff about OCS over at my blog here:
http://www.markc.me.uk/MarkC/Blog/Blog.html
It is a complex product I guess, but it doesn't have to be.
I'd say it's one of the few MS products you can't just throw the DVD in and get on with it - I.e. you have to put a bit of planning into what you're doing, and understand what you're trying to achieve - and what bits of technology do which bits.