I've only ever tried Modalert. My only comparison as a cognitive enhancer is methylphenidate.
Modafinil does help with concentration and cognition in general, especially when I'm sleep-deprived. It's a bit better than methylphenidate in that respect, and at the same time is not as stimulating, i.e. I don't experience a subjective surge in energy with modafinil. It does help with motivation in a different way - mental activity becomes easier and that ease of doing/learning things helps me to stay focused on them.
On the other hand, it gives me some bad side effects. Headache doesn't always happen, but often appears when modafinil effects begin to subside. My worst complaint, though, are digestive issues - nausea (always) and diarrhea (often). And a general feeling of physical sickness. Keeping my dose low helps, but effective doses always cause this to some extent. It can be distracting and it makes me hesitant to re-dose or take it repeatedly for several days.
I have to add that I know two other people who have tried modafinil and had no such complaints. One of them is prescribed methylphenidate and likes modafinil much better. Overall, I personally find methylphenidate to be a preferrable cognitive enhancer simply because I don't experience those side effects with it. Unfortunately, I can't get my hands on it as easly as on modafinil, so the latter one often remains my last resort (I'm often having bouts of insomnia and wouldn't be able to get much work done without pharmacological help).
I'd love to find out if the original brand of modafinil really works differently, as some people claim.