AppCode has been a wonderful tool for me. I have been using it since the EAP of 1.0. If you've used IntelliJ IDEA before you will feel very comfortable here. The refactorings are great and only getting better. The debugger and the ability to inspect variables much more easily that in Xcode is awesome. I just recently renewed my support on AppCode and I could have saved some cash, but the tool is so inexpensive as it is that I don't care. But, I am going to buy my upgrade to IntelliJ IDEA 12 today. I've had every version of IDEA since version 2!
But, again AppCode is an awesome tool. When you consider the only way JetBrains can write a tool that reads Xcode projects is to constantly reverse engineer what Apple puts out, it's quite amazing.