Personally I think the Async/Await model is too limiting. E.g. lets talk about this:

    var result = await client.GetStringAsync("http://msdn.microsoft.com");

From the perspective of the caller, that code is still blocking. It solves the problem of threads being blocked, but that's only one of the problems you have.

The real and more complete alternative would be to work with a well designed Future/Promise framework, like the one in Scala [1] which is also usable from Java [2]. Doing concurrent computations by means of Futures/Promises is like working with Lego blocks.

Let me exemplify with a piece of code that's similar to what I'm using in production. Lets say that you want to make 2 requests to 2 different endpoints that serve similar results. You want the first one that completes to win (like an auction), but in case of a returned error, you want to fallback on the other one (type annotations and extra verbosity added for clarity):

    val url1 = "http://some.url.com"
    val url2 = "http://other.url.com"

    // initiating first concurrent request
    val fr1: Future[Response] = client.get(url1)

    // initiating second concurrent request
    val fr2: Future[Response] = client.get(url2)

    // the first fallbacks on the second in case of error
    val firstWithFallback: Future[Response] =  
      fr1.fallbackTo(fr2)

    // the second fallbacks on the first in case of error
    val secondWithFallback: Future[Response] = 
      fr2.fallbackTo(fr1)

    // pick a winner
    val finalResponse: Future[Response] = 
      Future.firstCompletedOf(firstWithFallback :: secondWithFallback :: Nil)

    // process the result
    val string: Future[String] = finalResponse.map(r => r.body)

    // in case both endpoints failed, log the error and 
    // fallback on a local copy
    val stringWithFallback: Future[String] = string.recover {
      case ex: Exception =>
        logger.error(ex)
        File.read("/something.txt")
    }

Given an HTTP client that's based on NIO, the above code is totally non-blocking. You can do many other crazy things. Like you can wait for several requests to complete and get a list of results in return. Or you can try the first url and if it fails, try the second and so on, until one of them succeeds.

In the context of web apps, you can prepare Future responses this way and return them when they are ready. Works great with the async responses support from Java Servlets 3.0. Or with a framework such as Play Framework 2.x you can simply return Future[Response] straight from your controllers [3]

[1] http://docs.scala-lang.org/sips/pending/futures-promises.htm...

[2] http://doc.akka.io/docs/akka/snapshot/java/futures.html

[3] http://www.playframework.com/documentation/2.1.0/ScalaAsync