One small thing came up my mind to bother me about RESTClient add-on is that can't render "Response Body (Preview)" when HTML returned from server. There is a small negative left padding exists somehow. Except that bug, it's flawless.
Update: Rating decreased 5 stars to 3 stars due to a few bugs/inabilities; * You can't add more than one headers properly. * No raw view for HTTP request (as Medlock Perlman mentioned below). I hope developer will consider these comments for next release.