As the project lead of ServiceStack I can list some features of the ServiceStack Service clients:
The ServiceStack Service Clients are opinionated in consuming ServiceStack web services and its conventions. i.e. They have built-in support for structured validation and error handling as well as all clients implement the same interface so you can have the same unit test to be used as an integration test on each of the JSON, JSV, XML, SOAP and even Protobuf service clients - allowing you to easily change the endpoint/format your service uses without code-changes.
Basically if you're consuming ServiceStack web services I'd recommend using the ServiceStack clients which will allow you to re-use your DTOs you defined your web services with, giving you a typed API end-to-end.
If you're consuming a 3rd Party API I would recommend RestSharp which is a more general purpose REST client that is well suited for the task. Also as ServiceStack just returns clean DTOs over the wire it would also be easily consumable from RestSharp, which if you prefer its API is also a good option.
UPDATE - Using ServiceStack's HTTP Client Utils
ServiceStack now provides an alternative option for consuming 3rd Party APIs with its HTTP Client Util extension methods that provides DRY, readable API's around common HttpWebRequest access patterns, e.g:
List<GithubRepo> repos = "https://api.github.com/users/{0}/repos".Fmt(user)
.GetJsonFromUrl()
.FromJson<List<GithubRepo>>();
Url extensions
var url ="http://api.twitter.com/statuses/user_timeline.json?screen_name={0}"
.Fmt(name);
if (sinceId != null)
url = url.AddQueryParam("since_id", sinceId);
if (maxId != null)
url = url.AddQueryParam("max_id", maxId);
var tweets = url.GetJsonFromUrl()
.FromJson<List<Tweet>>();
Alternative Content-Type
var csv = "http://example.org/users.csv"
.GetStringFromUrl(acceptContentType:"text/csv");
More examples available from the HTTP Utils wiki page.
