Have look at budypress.org and look for the example site as well as the demo site
I do know of an effort in our own organization where they have used the LifeRay portal to build a client engagement community. We have a portion of our consulting business that focuses more on products (vs. solutions) and this group is finding this community very helpful. I should mention, they validated for us the notion we had that using open source meant that you need more development staff to install, build and manage releases. They are constantly trying to get new pieces working.
As the C3 planner (C3: Connect, Communicate, Collaborate - our branding of Jive deployment), I was aware of this other effort in our company - and like all big companies, sometimes groups need to solve their needs as quickly as they can without waiting.
I guess I'd say, to a certain extent this decision shouldn't be a technology decision, but rather a business decision. For our team, we were really clear on the requirements and I feel we knew our stakeholders and sponsors well. We accurately judged their appetite for the trade offs in the triple constraints (time, cost, quality). Open source is great for many. But we knew what our guiding principles were - and as we went through our RFI process and proof of concept - Jive came out the clear winner. (We can say that now - since our pilot is live and even announced in the marketplace.)