Is Demo still good/useful?
Yes Demo still has some use, but of course is not as useful as Skelectica or Lavanica which are the more recent "Event Only" dragons released.
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Demo is still a good hero in a vacuum, but when compared to how he is attained, he loses a lot of value.
Since, Demo cannot be realistically obtained for free, money must be spent. It's pretty easy to find him for sale for 20 dollars.
However, 20 dollars can get you a whole lot of resources these days.
So while I think he is a good hero, I do not think he is always worth the cost in comparison to what 20 dollars can get you. Just a matter of time till he's 10 dollars.
Although not as strong as he used to be, igg de nutted him with one of their updates a while back, he is still a strong player. My setup is devo, fully upgraded skin, 8 FG, 7 SL, 4 VL and all accuracy traits. Get him with a buffer like PD or mike and he is a great sniper. At times he can still devastate an area on the play board.