I got it for Airplay and some of the iCloud integration stuff my wife uses all the time. $20 for both our computers isn't too concerning.
I have not noticed slower performance (like we did with Lion initially) and I really like the notifications and improvements towards seamless integration of my wife's devices/data with her laptop. I have study groups at my house a lot, so Airplay is the nice feature for me. We have moved to the living room since I can now put up our powerpoints on the TV quickly and easily.
I usually go ahead and update software when it comes out anyway. New security updates, regular software updates, etc. always seem to run without issue when you have the latest version of OSX. I think the iOS stuff is stupid, so it takes me about 10 seconds to remove it from my tool bar and pretend it never existed.
FCP7 and all of the programs associated with it are still fine in every way for those who are curious.