The most important advantage of low cost airlines is the price of the airline tickets. Thanks to quite high number of lowcosts there is a strong competition which makes airlines lower their prices even more. That's why you can often see airfare deals and special offers.
On the other hand there are many disadvantages. One of the most clear is the level of comfort which is not very high. In most cases you don't get any free refreshments onboard. Moreover, you have to pay for your baggage and there are many small „hidden“ fees.