It's interesting but not very practical. It's an "algebra of types" with only constants, no variables, since C does not have type variables. Thus it misses much of the point of algebraic data types in a language like Haskell (deep composability and generics with very little code to write).
Take one of the most basic sum types in Haskell's base library, Maybe:
data Maybe a = Nothing | Just a
This simple construction gives you the ability to write functions over optional values and avoid the issues with null pointers. This would be an amazing feature to have in C but it can't be achieved due to C's very limited type system.
I used to define `Maybe` in C as a macro over a tagged union. It really unveiled a number of logic errors at compile-time, but the issue of course is to use this kind of metaprogramming sanely. E.g., instantiating `Maybe` to some other macro `T(a)`, where `a` is some other (concrete) type, would already cause a headache.
In the context of C99, isn't tagged union the more usual name instead of sum type?