I like it so far.. is it radically different change? no.. but if you are bored , $20 is nice fun to go for... you blow that in couple can of beer these days at the bar..
I cannot think of any one feature I will use it all the time(air mirror? even though I have apple tv.. that's bleh... notification center? seriously?.. One place I don't want to get constantly interup is my imac and mba.. I have iphone for that..
Safari change came with the browser upgrade itself.
imessage? it's not even completed yet(in terms of unification and also the way it doesn't work well all the time yet).
But.. I like it.. I am finding myself installing over imac this morning .. doing it now(I did mba last night).
It just feels fresher and little bit more smooth.. you are apple fan.. go for it.. why not? At $20 bucks.. it's nice chagne(mine came w/ 2012 mba purchase)
Also, if SL has been working for you your computer is bit old but you don't care(NOthing wrong w/ that btw).. which means you should be fine.. This upgrade is for people who are looking to change their computer little bit for little price.