Its hard to say anything about spring without seeming biased in favor. Spring's RestTemplate is as solid as can be expected. Their standard call back based approach for safe release of resources works well even for the REST template. If one is using Spring-mvc and their REST support, very few reasons would drive me to consider an alternative framework for the client. One among them is definitely finer grained control, another is Http Client 4.X support. Documentation is sparse on RestTemplate as well. But one has a community to back up on. There might a bit of up front customizations, standard call back etc that an adopter might create but once done I feel that it would be a easy to work with the RestTemplate.
Clearly one has many choices in selecting a client side framework for RESTful HTTP. In most cases it probably makes sense to use the same framework for the service and client end. Then again, if you are only a consumer of a service you have multiple choices among those shown above as well as the option of using Apache HTTP Client or Components directly and bypassing the higher level frameworks. For some, integrating with the spring framework is important and all the above frameworks have means of integration points, both on the service and client sides. Support for Client Proxies is something one might want to consider as they tend to simplify the programming model. Further if Resource definitions can be shared among client server, that can be quite useful in being DRY (Don't repeat yourself) and provide means for contract definition. For those interested in performance and tuning of the HTTP Connections, using a framework that allows you to manage connection pooling and other parameters is definitely the way to go.
For more information on Spring RestTemplate, visit Spring blog
Spring RestTemplate Example: