They look like convenient additions... but I'd be reluctant to add so many category methods onto a Foundation class. Especially for un-prefixed methods.
If a future version of iOS adds an -sum method, for example, you'd better hope it has the exact same semantics as yours, otherwise if the system calls -sum expecting its implementation to be used, bad things (or worse, bad things with no apparent cause) will happen when your implementation is used instead.
And you changing the category won't help the versions already out in the wild. Hopefully, automatic updates will mitigate this to some degree but not everyone will turn them on.
There are workarounds (like prefixing methods with a pseudo-namespace), but they're all a bit ugly. Might be worth considering though.
If a future version of iOS adds an -sum method, for example, you'd better hope it has the exact same semantics as yours, otherwise if the system calls -sum expecting its implementation to be used, bad things (or worse, bad things with no apparent cause) will happen when your implementation is used instead.