I agree with most of the answers, but I guess it will be helpful if summarizing them.
反而, it can be used to describe a reverse fact, no matter positive or negative.It usually indicates sth. beyond expectations. Quoting the examples from Yvonnewu and smile:
反而，反过来 can be replacements, but you need to be careful when replacing them, because they are very likely in different sentence structures. Examples:
我帮了你，你反而责怪我。==>我帮了你，你 [却/还] 反过来责怪我。(Add 却 or 还, when using 反过来. 却 makes the sentence sounds more formal, while 还 is more colloquial.)
To my understanding, there are some cases when they can not be replacements, or it sounds very odd if you replace then.
1) The case it is better to use 反而---when indicating a neutral fact.
哥哥很矮，弟弟反而很高。 If replaced---->哥哥很矮，反过来，弟弟却很高。It sounds wordy to me.
2) The case it is better to use 反过来--when expressing "in turn" "the other way round" or vice versa.
2.1. when the two things are in equivalent status
理论的基础是实践,反过来理论又为实践服务.（Theory is based on practice and in turn serves practice. ）
反过来也是一样.（ It's the same the other way round. ）
2.2. when involving two parties, it is like a "revenge".( Party A hit back at Party B's action or remarks)
Mary指责Tom的粗鲁, Tom反过来指责Mary的懒惰。(Mary accused Tom of being rude, Tom in turn blamed Mary for her laziness.)
Hope that helps. Better refer to a dictionary. There are far more examples that I could list. :)