Considering how few people acutually use online help or guided tours to learn how to use software, the Office Assistant or technology similar to it, may be a very useful way to help people learn about software features they might not otherwise know or learn about. The tick is to implement it so it does not anoy the people who don't see it as being useful. Your question of the design of the instructional interface is an interesting one. I recently looked at several CBT's and WBT's and was suprised by the high degree of difference in the design and presentation of the navigational components. With one I have a very difficult time finding how to get to the next topic.
Some systems seem to over emphasis a cool design, while ignoring the basic usability of the system.
For me, an EPSS is primarily software that helps people perform real tasks efficiently, effectively and accurately. The software may contain various perfomance support features, like the Office Assistant, online help, tool tips, etc. In some instances an EPSS may contain an Instructional performance support element, but I don't think instructional system are that useful for helping people learn to use software, rather they may be most useful for showing people how to perform tasks that are performed outside the computer mediated environment. For example they may be most useful in a equipment maintenance EPSS that enables a person to diagnose a problem and then use an instructional component to learn how to perform mechanical repairs.