While good for most conversions, it misses the most essential aspect of lengths in imperial, which is that feet are given in feet/inches (for example, a meter is just over 3 foot 3.37 inches). Unfortunately, this add-on gives the conversion only in pure decimal form, which is awkward and inconvenient. It gives a meter as 39.37 inches or 3.28 feet, neither of which are very useful. Likewise if you wanted to go in the other direction but were given something like 8ft 7in, you'd have to whip out a calculator just to figure out what you're supposed to put in for feet or inches, but at that point you may as well keep a conversion chart on hand and just do the math yourself.

Otherwise, this is perfectly functional and quite useful for very big or very small units, but base 12 units aren't really handled the way that they should be.