QFT
You do not want to be the guy that chose the OSS that FUBARed something important. You want to be the guy with the phone # for the people that are supposed to fix it because you paid them.
Also large one time expenditures on capital can be accounted for differently and save a company a bit of money vs. OSS + tons of problems even if the out of pocket expenses are the same
Yeah, it pretty much boils down to functionality and time. Open Office doesn't support 100% of the functions that Microsoft Office does, and you might get a lot of people complaining about it, especially the math-types (engineers etc.). Plus there's re-training time from regular Microsoft Office...people get really used to a certain version & layout; even though it sounds like a simple upgrade, for a lot of people it's a huge change in their day-to-day lives. Even upgrading regular old Office has been problematic (oh the complaints I've heard about the new Ribbon!).
The biggest thing to look at is your time investment. I'm a really big fan of OSS, but like Legendary said, ultimately you can be the guy who fixes it, or the guy who has the phone number for the people who are supposed to fix it. I switched to a large company as sys admin this year and had a whole list of OSS-type projects I wanted to implement, but I just have no time to do them - my day is all about time management. Sure it's all pretty easy, but right now my to-do list is over 3 pages long of simple things and some days I only get 2-3 things on that list done due to the length of time each project takes and the interruptions I encounter during the day for small "emergencies".
So be aware that you will have to become an expert in Open Office and start fielding phone call after phone call and email and email. OSS is a great thought, if it meets the company's needs and if it's something that is manageable.
So to recap 3 primary things to be aware of:
1. Paid products have paid support, meaning you're off the hook for support. OSS sometimes have paid support and mostly don't, which means YOU are the support, which is a time investment.
2. New, non-standard products require re-training, which is also a time investment. It may seem like a simple upgrade, but it won't be. Whenever we do major upgrades at my sys admin jobs, I usually spend the next month or two hopping around answering user questions (no companies really want to spend time/money training people on the new software, so they don't, so it's all back to you again).
3. Functionality usually isn't 1:1, which means you'll be missing features, some of them critical. So you might have small pockets of people who need Microsoft Office, so you'll have to support that too.
If you enjoy it as a hobby (i.e. don't mind investing the time to learn it at work) and you have the time to do the support required, it's a pretty sweet deal. Otherwise be aware of the time problem and the user-frustration problem.