A View from Over Here - The Case *FOR* VOIP

In a well reasoned article entitled The case against VOIP author Carl Weinschenk makes three cases against VOIP:
1.) Companies looking at VoIP primarily as a way to cut costs on voice services should be careful.

I agree that in VOIP, as in all aspects of business, you should explore the full cost of any product or service prior to purchase. In the case of VOIP, the customer who we have seen with HUGE savings are those who think outside the box and take advantage of VOIP to its fullest extent. For example, worry less about the per-minute savings from VOIP and look at the savings you can obtain by sending the entire help desk staff to work from home thereby saving rent on an entire floor of people. One of the major advantages of VOIP is that it is not tied to a single geography like land lines. Users can be anywhere and still have full phone system access.
Reality: The real cost cutting comes from leveraging VOIP to meet your business needs, not just saving per-minute costs.

2.) The underlying plumbing counts. Organizations must do pre-deployment assessments, and those antiquated or inadequate infrastructure should not deploy VoIP. Upgrading substandard networks can wipe out any savings from VoIP.

I actually agree with this. But, if your network is some ancient token ring, then a little YouTube is going to bring you to your knees as well. Any decent 100Mb LAN is more than adequate for VOIP. Upgrade now and use collaboration tools like WebEx and other great business tools.
Reality: If your network isn't ready for VOIP, it's not ready for most of today's multimedia intense Internet applications and you need and upgrade anyway.

3.) There are many organizations that can get by quite nicely without VoIP. IT and finance should have a clear understanding of why VoIP is being deployed and a detailed and clear-eyed costs/benefits analysis should be performed.

You should always have a clear understanding of why any initiative is being deployed and a detailed analysis should be performed.

Bottom Line
VOIP is a perfect choice for any size company in a greenfield (new office, new network, new pbx) environment.
VOIP is a great choice for small companies looking for more functionality from their phone system. With VOIP they can do more (auto attendants, voicemail) with less.
VOIP is a great choice for companies with branch office and/or remote workers. VOIP is not tied to a location and allows for easy collaboration.
For large companies all in a single office building with no remote workers and no plans to let anyone work from home, VOIP must be evaluated carefully. If the only thing a company is doing is unplugging the phone lines and replacing them with a broadband connection, then the value of VOIP is diminished.

Lucky for us, most companies fall in the first three categories.