Efficient string concatenation?

[JR]

On Friday, 15 November 2013 at 22:33:34 UTC, Brad Anderson wrote:
> Appender in std.array is probably what you are looking for.  
> std.algorithm.joiner is also useful (no allocations at all 
> even) but the use case is a bit different.

Is Appender considered up to Phobos' current standards? I vaguely 
remember reading something about its internal memory management 
being worthy of critique.