Personally I have been enjoying CS:GO, that being said is it a vast improvement over CS:S? Visually: Yes. Gameplay: Meh, I mean they have fixed a few things while at the same time creating more problems.
Worth $15? Well seeing as CS:S retail was 29.99 (box) back in the day, and still is 19.99 (29.99 for anthology) then I will say yes, for the gameplay experience it is very much worth the $15, heck I would even say it would do well at the same price as CS:S (19.99)
The problem that plagues it right now is not very many maps and not very many player models, but I think this will soon change. It seems Valve wanted to make CS:GO a bit more community run (Map making, skins, etc) like it has been doing recently with its other game (see TF2 and Dota 2) and I wouldn't be surprised to see "workshop" open up for CS:GO soon-ish.