Hey. I would like to give my opinion about activity stuff.
1. Great idea poor implementation.
I have few issues while using activities.
The hard way to switching between activity is the worst.
Swithing between desktop is much better, much better implemented. nice animation, great to use pager. different configuration for each desktop makes it more usefull that activities. One Think that I only miss with desktop switching is power managment per each desktop like for each activities. I mean something like function to set for second desktop (multimedia desktop) no screen saver, and no monitor off.

2. A number of desktops for each activities.
I have two activities. one normal for everyday use. Here I use 4 desktops (called: desktop-with folder view of my real desktop, downloads- with folder view of my downloads filed from FF or RKQ, movies- for my catalog of films and sitcom, university- my stuff for study)
And second activity for watching movies online throught firefox, rekonq-- And I need it only because I can set here to not turn off the screen or to not run screen saver or other stuff that could break my "theater time" or "presentation time"- It is quite annoying, You have to admit me at this point...

3. And my last thoughts is that I think That the activities implementation should be dropped. And all actual utilites and functions should be implementet to "desktops" stuff to increase their usability. Please give my Your thouts, maybe some improvement clues.

I know that the plasma active use a lot of activities stuff. And in near future it will be the part of home desktops and laptops computers.
But I just like the desktops much better that this activities, however swithing activities in plasma active is really really really great. I cant wait to see it implemented in desktops kde.