Indeed, but it's definitely a hardcore pay-to-win game (at least ladder definitely is).

If you play well, you get to a point where your opponents' cards are way overstatted against yours. And that's where the temptation to give in and make a purchase comes...

Yes. Core mechanic is incredibly fun and it starts when you start playing. Cards you get by reaching higher ranking are not necessarily more fun to play and you don't win more with them because you are playing always with players of similar strength to you.