They won't build the champion for you, if that's what you're wondering. They give good suggestions sometimes, and bad suggestions other times. Should go into a custom and make sure you're aware of all of your viable options, and know what to build and when to build it whether you pay attention to the recommended or not. You could also boot up the legacy client and dig through there to theory craft for awhile.
Are you against an AP heavy team? AD heavy team? Targeted CC heavy team? Non-targeted CC heavy team? Can you outplay them? Are you ahead? Behind? Who's fed on the enemy team and what should you build to counter them?
It seems like a lot of questions, but it should only take a minute to figure all of that out, and being able to flex your build to the situation is the difference between winning a fight with 50 hp, or getting wiped and falling further behind.