Learn French: Ce mec est vraiment sympa. - This guy is really nice.

Yes, mec is an everyday colloquial word for "guy" or "dude". It is extremely common in spoken French, but it is informal, so you wouldn't use it in a formal essay or a strict professional setting. For a more neutral word, you would use homme (man) or garçon (boy).

Is sympa a complete word, and does it change if you are talking about a woman?

Sympa is the shortened version of sympathique. French speakers love to shorten words, and sympa is used much more often than the full word in daily conversation. Because the full word sympathique ends in an "e", it is the same for both masculine and feminine nouns. The short version sympa works the same way: you can say ce mec sympa (this nice guy) or cette fille sympa (this nice girl).

What is the difference between using sympa and gentil for "nice"?

While both translate to "nice" in English, they give off a slightly different vibe. Sympa describes someone who is friendly, cool, and generally pleasant to hang out with. Gentil, on the other hand, emphasizes sweetness, kindness, or generosity (like someone who goes out of their way to help you).

Why is the word for "this" ce here instead of cet?