For me the semantic desktop was too resource hungry so it was turn it off or move to a different DE - at one point I had to remove the actual binaries in order to disable it the devs were so keen to push their ideal DE it seemed.
TBH if I'd been using the full KDE communication suite then perhaps it would have made sense but the refusal to support the sending of html email for kmail (did they change that yet) moved be back to Thunderbird a long time ago.
I applaud the innovation and feel that the community would have really been behind it if it had been optional from the get-go. Having never used Activities I still appreciate that KDE devs tried something different, the great thing with Activities is that – whilst initially you couldn't turn them off – they kept out the way and used little-to-no resources.
The desktop search, integrated to Dolphin, is great. Usually I use locate/find/grep but I'd use an integrated search if it could be tamed in its resource usage; but that's really the limit of the "semantic desktop" that I find useful under my desktop use at present.