The chances of bricking your camera are extremely low (but not zero).
I think something not everybody understands is the Magic Lantern does not install into your camera but to the SD card. My understanding is that the only change that is done to your camera is to enable the "bootdisk" flag, which is a very minor change. This flag tells the camera if it should try to load and execute software stored in the SD card.
Magic Lantern enables the bootdisk flag in your camera and then installs to the SD card. The bootdisk feature is what enables the camera to load Magic Lantern at boot time.
This is important to know, because if you replace the Magic Lantern SD card with a regular SD card (one that does not have Magic Lantern installed) or no SD card at all, the camera will not find any code to load, so it will just load the stock firmware, and there will be absolutely no trace of Magic Lantern. So you always have the choice to run Magic Lantern or not if you keep track of which of your SD cards have Magic Lantern installed and which do not.
My experience with Magic Lantern on my 60D has been very good, I think the benefits far outweigh the disadvantages and risks. Note that the software does crash from time to time, and sometimes it is necessary to remove the battery to reset the camera. I haven't experienced any crashes that didn't clear when removing the battery. The recent 2.3 release has been extremely stable in my experience.
If you want to minimize the risks I recommend the following:
- Follow the installation instructions to the T. Read all the instructions and understand the entire process before you actually do it.
- Only work with stable releases. At the time I'm writing this the stable release is 2.3.
- Don't be an early adopter, wait a bit before you jump on to the new release. Let the more adventurous users (I'm one of them) try the new releases first.
- Read the Magic Lantern forums to find out of potential problems other users are experiencing.
- Always have an SD card w/o Magic Lantern installed at hand as a backup plan.
Important information for Eye-Fi card users:
- An Eye-Fi card cannot be used with Magic Lantern (this is documented)
- An Eye-Fi card without Magic Lantern cannot be used as a backup card (this is not documented as far as I know). The camera will not boot and appears to be broken if you try.
- In order to be able to use an Eye-Fi card again: Remove the bootdisk flag using Magic Lantern sw (i.e. full uninstall)