Excel is capable of doing almost anything and there are myriad ways it can be useful. The question to ask is how to get people started. Like anything else, it needs to become second nature--an extension of yourself--to be a useful tool. You probably can't envision working without a calculator, but if you had to look up how to use it every time you needed it, you probably wouldn't use it. Excel will be useful when the user can do the basics without thinking about the mechanics. From there, they can explore other features that will make their lives easier.
Start with any basic tutorial or class to get proficient with the fundamentals and a general familiarity with what Excel can do. For features beyond the basics, focus initially on just recognizing the broad range of functionality that is available--what can be accomplished--rather than the details of how to do each thing. From there, people can easily learn more advanced features as they have use for them. Most things that someone other than a real pro would use can be learned through readily available examples and tutorials if someone is familiar with the basics. It will be especially helpful if one person with an interest can become proficient and be the in-house guru that people can go to with specific questions.