A lot of languages (Java, C#, Go, etc.) have very inexpressive type systems, and that tends to make many dynamic language developers wary of them. Contracts has a very expressive type system though. It understands duck typing, adhoc union types, and a bunch of other stuff that makes it a great fit for a dynamic language.
Plus, we've disabled it in production, so we get coverage in test/development mode, but no performance penalty in prod. I look at it as an extended form of testing. Type contracts are very quick and easy to add, and they help tests catch additional errors. They're also useful in code reviews. Sometimes I'll have to search for 5 or 10 minutes to figure out what kind of thing is getting passed in, where the contract makes it perfectly clear.
The best part is that the contracts are optional. If some method takes wildly differing arguments and it's going to be a pain to give it a contract, then don't.
This is the best tool I've added to my Ruby arsenal in some time. I strongly recommend taking it for a spin.
