I will add that if it identifies two files as the same movie, it will put them together. You then have to split them if you want them to be two different movies, and the one that is wrong you need to do the "fix match".
You can set the metadata scrapers used for artwork, and even add a hierarchy for which ones to use first. And if it isn't right, you can edit and select cover art and poster, or as Matt said, you can always get your own cover art and drop it in.
For naming, I have had very few issues. All my stuff is in one folder, and I sometimes use underscores, sometimes dots, sometimes spaces. I put the movie name followed by the year (sometimes), which is sometimes in parentheses and sometimes not. The year helps for sure. Maybe one in 25 get matched wrong, and maybe one in 100 gets really messed up, can't be found, or gets matched with an existing movie and doubled up.
Casino Royale 2006.mkv
Casino Royale (2006).mkv
All these will work just fine.