Coming from the US, this system is actually very good. I stayed in Las Condes, and traveled to city…read morecenter almost every day.
You purchase a Bip! card (about $2.50), which stores money value on it, and you can reload the card at any station, via person or machine. You tap the card on the turnstile machine upon entrance only.
There are 3 price levels: rush hour, regular, and low. The difference between rush & regular is only 10 cents or so. You are charged for each ride (660 pesos, about a dollar), and there is no day pass or week pass. This is a bargain compared to the US - but you have to plan your trip since each trip will cost you.
And yes, this system is extremely crowded most of the time. It's not really worth it to wait for the next uncrowded train because they will all be crowded. Trains run the entire length of the station, and are pretty frequent during the day.
I mainly only took the Line 1, since it served my area and downtown. Most signs are in Spanish & English, so you can get by if you are good with directions. I never felt threatened or unsafe on the metro. There is a security presence at most stations near the turnstiles.