I used Spivak in my calc class in college, and it was a decent text. Having the answer key is extremely helpful, especially with the later chapters. There are some methods of proofs that are needed to solve the problem that aren't discussed at all in the text, and you are unlikely to be able to come up with them on your own.

Obviously, don't use the answer key unless you've spent a significant amount of time trying to find the answer yourself.