Custom firmware is for special and unusual shooting applications. For instance, if the photographer shoots a lot of 'low light' pictures then some sort of custom firmware designed especially for low light conditions would be the way to go. The standard firmware would be suitable for general use under most conditions.
I have not personally used any custom firmware.
There are updates for the regular firmware and these should be installed for better performance of the camera. This is very similar to PC monthly updates. The camera firmware updates must be obtained from the camera manufacturer's website. They normally don't send notifications regarding these updates (if any).
You can read that first, it is up to you. In the FAQ section, you can see it is legal and but it may or may not void the warranty. Also they (Magic Lantern) said they can't be certain that it's 100% safe.
If your camera works fine I will recommend don't try it. Just stick with the canon firmware. If something happens to the camera you know what will come next.
I have a Canon T4i and recently downloaded the Magic Lantern firmware and installed it on an SD card. I had to download a beta version as there isn't a "final" version available for the T4i. I haven't had a chance to use it much yet but will say that it has unlocked features not previously available. With the T4i I have to press the trash can icon to get the Magic Lantern firmware to load from the SD card. You will know when the Magic Lantern firmware is loaded because there will be a message at the bottom of the menu screen with the date and version of the firmware. I took a risk by doing this and so far so good. It was non-destructive and the latest Canon firmware update is still in place.