The most engaging messages are slightly unique in nature. They combine a mix of clever scripting and music. Slight humour is often used to promote a product or service and is usually quite effective. It's also very important to choose both the written and spoken tone that appeals to the characteristics of your audience. If your target market is the younger generation, a script that includes slang words and is quite laid back would be an excellent choice. If your target market is primarily business professionals, then a more articulate choice of words would be more suitable.
The business answering machine greeting you use should be fairly short and to the point, though you can use it as a way to demonstrate what your business has to offer. Your overall tone in the message should be professional and indicate to callers that though their call has been missed at that moment, they are important and you plan on contacting them as soon as possible. Within this, however, you should not indicate that you are talking to someone more important than the caller. Your business answering machine greeting should also be customized in any way you deem appropriate for your business and needs. Jotting down the greeting will prevent mistakes when recording it.

If they are calling without an expectation that you always pick up (such as if you are a business coach or a one-man shop) then thanking them for calling might make the most sense.
Length – if your callers typically wait 2 minutes at a time, then at least 2 minutes of programming should be the goal. The idea is to ensure the callers don’t hear repeated messages every time they call. A typical on-hold message is 4-8 minutes in length.