Yep:
Advantage for OpenOffice:
Export to PDF function allows you to create VERY portable documents easily when formatting and structure are important. PDF is the defacto standard for technical documents on the web, and is something that printing shops can easily deal with.
Using Microsoft Office it's file formats are not portable to other applications or older versions of MS Office. Sometimes you will have issues porting documents from earlier versions of Office to newer ones. If you want to have people receive documents created in Office you are basicly forcing everybody that you deal with to own your version of Office or newer versions. Also the propriatory format may cause big big headaches 5-10 years down the line if they need to be used again for any reason.
The HTML code that is generated by O

rg when making webpages with it is actually decent and fairly usable. Web code generated by MS Office is not.
O

rg is cross-platform and Windows is not. In order to use MS Office you have to be using Windows, and everybody you come in contact with has to use Windows. (there is a Mac version of MS Office, though. And you can have certain versions of MS Office working thru Wine compatability.) Office works on a very wide veriaty of operating systems and platforms.
You can make copies and give them away, and have friends and family download it off of the internet so that they have 100% compatable apps without making them pay for liscences or breaking the law.
disavantages:
It's not a Microsoft app and thus some people will simply ignore it.
It has slower startup.
It lacks a Access work-a-like that is aviable in the more expensive versions of MS Office.
Not 100% compatable with Microsoft formats. MS makes sure that MS products work together, but don't provide enough information to other developers so that they can acheive the same level of integration. So if you use something like Excel spreadsheets that work with MS SQL database you will have a tougher time translating that to work with O

rg's spreadsheet stuff. (athough O

rg does have database functionality so that it can work with SQL databases such as PostgreSQL and MySQL. Also there is a limited sort-of like Access app hidden inside of it...)
see here
O

rg doesn't compete with MS Office on a bullet point feature by feature comparision. (although most people dont' use most of those features or even know they exist, or would even care about them if they knew) Which can cause issues for more experianced MS Office workers that have a bunch of tricks and shortcuts that they are used to.
There is some extra work sometimes involved with setting up dictionaries and such for O

rg.
My biased conclusion:
Basicly unless you have a specific task that requires MS Office for some strange reason, O

rg is preferable. If your using a business that uses MS Office a lot, then it may be to distruptive to move everybody off of MS Office and that expense may counteract any reduced cost you get from moving away from Microsoft products. I think that most everybody can benifit from having it installed or making it aviable to people. If your have a business that needs productivity apps and you do not have either in wide spread use, then choose O

rg. You can always pay for MS Office later if you realy need it for something, which is not usually the case.
edit:
Although O

rg is designed as a entire "Office Suite" of office productivity tools there are other very excelent productivity apps that concintrate on one program at a time.
Such as:
Abiword: WYSIWYG Word proccessor.
GnuCash: Personal and small business financial/accounting software. (not aviable for Windows, unfortunately. Unix only so far (I think): OS X, Linux, Solaris, AIX, FreeBSD, etc)
Gnumeric spreadsheet software. (I don't know if it will work with Windows. They will have a version eventually)
KOffice, Office tools for the KDE desktop in Linux/*BSD
even up to a cross-platform EPR/CRM database software
Compiere.
(sort of like PeopleSoft/Oracle Apps... It'll track inventory and also provide point-of-sale services and features for small to medium sized businesses. requires Oracle 10g and Sun Java 1.4.2. The clients can by any java platform (OS X, Linux, Windows) and server can be anything java and oracle databases will run on (Linux, Windows NT/2000 server/2003 server)
Things like Abiword are more slick and much smaller then O

rg, and will open faster and run better on machines with limited resources. So if all you need is a word proccessor you may like Abiword better. It's a matter of taste. Things like GnuCash or Compiere may work out better for things that people use Access for,
( although there is a lack of free easy-to-use database front-ends for FreeSoftware/Linux stuff. However Access is mostly only usefull for systems that require a single user database. Not much functionality above a flat file storage system. For multi-user and networkable solution MS SQL with Access front end is pretty workable, though)