Real voices - best
OFP voices - next best
No voices - middle
Robot voices - worst
If you really cannot get real voices, then design your mission around this. It's perfectly possible to make a perfectly good mission with little or no dialogue. If you are cunning, you can design it around the voice files already included with OFP. There are quite a lot. I don't just mean the ones you get in triggers (which can be found in DTA/sounds.pbo) but the dialogue from the campaigns.
However, making voices is not hard. Bloodmixer's sound tutorial is a little old now but still gets you started. If your computer doen't have a microphone (and most do) you can buy a cheap one for $/Â£/â‚¬10 or thereabouts. Get your friends and family to act for you. Don't beg: bully. ;D Standard of acting doen't matter: even really bad acting is better than nothing. Experiment a bit to make the quality of the recording as good as you can.
Got no friends or family? Do them all yourself with different accents and pitches. We don't care how bad your accents are or how stupid you think you sound, it's still better than nothing.
Can't afford a mic? Go out and wash cars until you can. Borrow one from your uncle. Get people from the community to record the voices for you. Track down somebody from the community who lives near you and has a mic and go round to their house.
Native language not English? So what. Even bad acting in a foreign language is better than nothing, as long as you have titletexts in Engish. Better still design the mission around this. If you are from Ruritania get some Ruritanian addon units and make it a Ruritanian mission.
Robot voices, as well as being dreadful to listen to, just prove that you didn't care enough to do any of the above. They do not have the human emotion and inflection which is what having voices is all about. The point about having voice files is to hear somebody speak: with the robot you do not achieve that.
In other words, never, ever, use robot voices. IMHO.