We use it instead of M$ Office. It's free, and it does everything Office does. However, M$ Office has Outlook (the email program); OpenOffice has no email program. So, if you want something to do your email, then use Thunderbird or OutlookExpress or Eudora or something else...
Here. Lemme save myself some typing. In the rest of this explanation, I'll call OpenOffice OOo and M$ Office, just Office, ok?
OOo, depending on how you set it up, may be a little slower to start. (It isn't slower on a MacOSX system, but, I suspect you're running Windows, so there.)
OOo can create PDF files. Office can't. OOo can read Office .doc and .docx files, and create Office .doc files (not to mention of course .txt, .html, .xml, and some others), so you'll not have to worry about that. You can even set it up so it creates .doc files BY DEFAULT. This is likely what you'll want, if you want to trade docs with your friends via email, instead of just printing them.
OOo, however comes set up to create a file type called ".odt" which is an Open Document Type favored by several organizations including, for example, the European Union, the State of Massachusetts, and a growing number of others.
The OOo interface is slightly different. Some folks find it not as snazzy.
Add-ons are available for OOo (kind of like add-ons are available for Firefox, ya know?) So you can enhance its function, if you're into that sort of thing. And full documentation is available online, along with a forum of fellow users you can join to ask (and answer) questions of usage.
You may deduce, I'm an OOo bigot. So, take my advice with a grain of salt.