I don't know since which release activities are part of KDE. Maybe they introduces it already whit the first KDE 4 release. Anyway, on multiscreens it really works good since the 4.4 release series. Before 4.4, activities where a typical crash-scenario. Now especially for multi-head setups they made alot of improvements and bug-fixes. This includes the activities, but it's still not perfekt (at least for me).
Usually, activities are a nice feature. In my case i use an extra activity for showing all my system-information like cpu-temperature, cpu-usage, network-usage. Additionally i also have a weather-forcast plasmoid and one for the latest xkcd story :)
The problem now is that the activity manager shows 3 different activities. Since every screen counts as a "full" activity i could also just switch the left and the right screen. (in the past plasma would have crashed)
Here you can see what i mean:
But that's something which i don't want at all. Especially if i change activities via shortcut it can happen quite fast that i just switch the screens. Well, it's not a big problem (maybe it's a feature :D), but i guess this one could be better implemented...