SPY is my buy but I have rules. Buy and hold the SPY (S&P500 index ETF) has netted an average annual return around 10% since its inception about 1992. That beats the great majority of investment managers! That's just buying and holding! The downside is just the feelings in your stomach from the roller coaster ride. If you can manage that (or better, ignore it) nice ride. Clobbers inflation and beats CDs, not to mention bonds and online savings accounts (where I have most of my cash now).
I follow the 10/40 crossover method for buy/sell. Golden Cross = buy after 3 week confirmation. Death Cross = sell after 3 week confirmation. Some people use 2 weeks, others 3 weeks for confirmation. I back tested this system and since the SPY inception it's netted an average of 2% more than if buy and hold was your rule.
Of course, if you add to your holding or subtract from it, it complicates your system some. My testing did not account for those exigencies.
In late February I broke my rule and sold early saving me a lot of money... I just knew the market was gonna tank so I jumped the gun.