Saving seats is usually not a problem if the people saving the seats are only for one or 2 people but there are some people who take advantage of the situation and will save up to 4 to 5 seats in a row. This is abusive to the movie etiquette rules. If you are in a large group then those people should be there already in their seats, if they have already established the seats and they went to use the restroom or go get snacks then it is important that only a few people go and not your entire group. When a premier movie has opened and it is a packed theater people make a mad dash to find the best seats possible to suit their needs and when they see open seats and make a movement towards them only to have someone tell them the 4 or 5 seats in the area have been already taken this is extremely annoying.
Also you should never save seats around you just so you can have more room, use your best judgement and if it is a pack house then your personal bubble will need to be broken and you will need to share the area.
Unfortunately there is not much you can do about a seat saver except give him/her the stink eye and go on your way but if everyone can follow good movie etiquette the movie watching world will be a better place.