I think I'd just give up and go home if I ever saw this in code I had to maintain.

EDIT: Actually, looking more closely, I guess there isn't a performance penalty (I saw back_inserter and assumed it was required for the implementation). That said, I still think it's far too clever.

This might be one of the few rare justifications for overloading the comma operator, for syntax similar to that of Haskell's backquote:

    arbitrary+expr ,foo, arbitrary-expr

Even then, though, that would get ugly in a hurry when mixed with function calls, requiring parentheses for sanity.

Overloading the operator removes this sequence point. Too sketchy for my blood.

Some prior art: http://www.gamedev.net/blog/2/entry-19599-bad-code-here/

    vector3 a, b;
    float f = a *dot* b;
    vector3 c = a *cross* c;

http://en.wikipedia.org/wiki/Spirit_Parser_Framework

[1]: http://www.yesodweb.com/book/shakespearean-templates

auto result = "Hello" <repeat> 3 <join> ", ";

I had no idea what that might do. No expectations at all. Is this an attempt to write language X in language Y?

Really? How is it any different than: "Hello".repeat(3).join(",");

I read it as "repeat the string Hello thrice and then add a comma and space". It's not really clear that "repeat" returns a list.

A C# equivalent would be:

     String.Join(", ", Enumerable.Repeat("Hello", 3))

Using an Extension Method, it's easy to see how someone could mistake the output:

https://ideone.com/DuIvZN