Making a coaster that people will actually ride is hard as balls. It took me a really long time to figure it out. The number and frequency of inversions has a HUGE impact on what they will ride, so take it easy on those. I wouldn't build them as high as they can go either, because in my experience that never works. The mechanics are kind of fucked up though. For example, helixes must be ranked extremely high in the "excitement" category, because it's possible to make a coaster (preferably a giga coaster) that goes as high as it can go, does a bunch of small helixes all the way down, and then ends. Not only will people ride it, they will fucking LOVE it.
So yeah, if there's ever an issue with your coasters not making the grade, toss in some helixes.