A limit order only fills at the price you want, or better. Unlike a market order, it won't fill at a worse price. By using a limit order you avoid slippage.

Are limit orders A Good Idea?

Limit orders can help you save money on commissions, especially on illiquid stocks that bounce around the bid and ask prices. But you'll also save money by taking a buy-and-hold mentality to your investments.

Do limit orders take longer?

Limit orders guarantee a price, but you may not get filled until the stock price reaches your limit. Once orders are filled, they can take an additional couple of days to go through the clearing and settlement process, although you'll see them in your account pretty much right away.