I find __slots__ annoying. They seem to have a lot of rules about their use and make things like a borgs rather awkward to use (I even prefer to use a singleton instead of a borg in new-style classes because of this).
And all it does is safe some space in memory (AFAIK). Make me wonder if it's worth it. Google claims there are about 2k hits for __slots__. Compare to e.g. about 300k hits when doing the same search for __init__.
Maybe I'm just overreacting though. Maybe having all class data in one place is not that important. Or maybe there are just massive gains in using __slots__ for some people that do make it worth it. But I'm not one of them so hence my rant.