Alternate Data Stream as a method of steganography has the following pros:
1. Its FREE!
2. Its very easy to use. Check this out.
3. You do not need ant third party tools / software.
It has the following cons:
1. No support for passwords (unless you embed password protected files)
2. Runs only on NTFS (and Microsoft says that " Alternate data streams are strictly a feature of the NTFS file system and may not be supported in future file systems. However, NTFS will be supported in future versions of Windows NT. ")
3. Hidden data will be lost if you email the parent file or for that matter transfer it to a non NTFS partition / device.
While on the issue of Steganography, also check out steganography, the easy way... No steganography software required