It's useful when you want a sentinel object and you don't want to use None as the sentinel because it is a valid value. Some discussion here:

http://www.ianbicking.org/blog/2008/12/the-magic-sentinel.ht...

`is` is also useful for things like detecting cycles for walking object graphs.

But I'd estimate 99% of the times I've used `is` was for comparisons to `None`, and 90% of the rest of times for comparisons to other sentinel values.