Quote from deronwagner:
We often trade SPY with market orders, but only through ARCA, which will route to the best bid/offer. However, if sending a market order through either NYSE or AMEX, I can be pretty sure I will get filled at least 5 cents above or below the best bid or offer.
We do use ARCA stop market orders both for getting in AND out of SPY every day because it automates our trading process. Although I sometimes get a bad fill, it is usually pretty good and even the occasional bad fill is okay because there are too many times when I miss good trades because of trying to save a few pennies with a limit order. If I want to be in, I just want to make sure I am in the trade!
Beware, however, that bad ticks above or below the best bid/offer will trigger standing stop orders on ARCA, meaning that you will often get in or out of trades before you want to. Bad ticks do not trigger stop orders on NYSE, but then you will get a worse fill. So, it's a tradeoff.