Calculator X Free touts itself as an upgraded option over the built-in calculator already on your iPhone, but the plethora of ads take away too much from the overall experience. Ultimately, there are few reasons why it should replace what you already have.
There is no setup time so you can start using Calculator X immediately after installation. When you do, you'll see an extra column of options, including square root sign, percentage, 1/x, and an extra memory tab. You can also view the paper tape for your calculations, which is a very nice addition, especially with a built-in e-mail tool. Multiple times during testing, however, an ad would appear partially over the screen, hiding behind the numbers and blocking the output screen until removed. It was frustrating and could severely hinder productivity if you were using it for ongoing work.
The app, itself, works, and may offer a few extras, but for anyone not interested in trading aggressive ads for those upgrades, this app is a pass.