The first builds were complicated as we had to compile the source from dailies scattered about on various sites, now everything is handled by EOSCard - if you can update your firmware with the camera menu and have a CF card reader, you can install ML (yes, OS X users have a couple more hoops to go through b/c EOSCard is Windows-only).
The biggest risk is if you interrupt the firmware update process, that's the cause of most "brick" reports but even that is usually fixable. You have to be really pushing things to do any permanent damage (in theory some of the super-high-bitrate video modes could overheat the chips, and there's a theoretical issue with dual-ISO affecting the buffers but nobody's seen it happen). Is it as stable as Canon's firmware? No, hence the 'alpha' tag - but realistically the bits that work are working fine, and the bits that aren't (such as continuous RAW) are not enabled in the builds that most users will be downloading. A few modules come and go as they're fine-tuned (ETTR, etc), but the core stuff like zebras and histograms are as reliable on the 7D now as they are on the other bodies. If you only want to shoot raw video it's not in any way ready for you yet and I wouldn't bother installing it. If you're shooting stills or H.264, even something like peaking or the Magic Zoom window could be a game-changer.
If you don't want to risk setting the autoboot flag, you can still use the original alpha 2 .FIR file, and 'update' your firmware each time you turn the camera on. That version does not make any permanent changes to the camera whatsoever.
I've been using ML-7D on commercial shoots for months, never had a problem. If ML itself errors out, remove the battery. Even with the bootflag set, if you want to go back to vanilla Canon firmware at any time just use a non-bootable CF card without the ML files on.
Autoboot was solved on the 7D a couple of weeks ago and is now very simple.
I guess we have pretty different ideas about what 'very simple' means. That post is from yesterday, a more detailed and helpful one is here: http://www.magiclantern.fm/forum/index.php?topic=7464.0
Its prefaced by "this is a guide only for if you want the VERY EXPERIMENTAL and POTENTIALLY DANGEROUS autoboot version of the beta 2". Partly bum covering of course, but not complete hyperbole.
Things are progressing quickly and theres some very promising stuff happening, but also it means levels of testing in the wild are going to be much lower.