It is a LOT of fun to play. I've hit turn 5 Tyrions, Ysera unleashed and Hyreeks. It also makes each turn more fun as you wait to see what's in the box!
Often for every busted turn there is something unplayable like Elysiana. Saying that, even vanilla legendaries can be solid on turn 5 as a 5/5, e.g. Zilliax, Onyxia, Velen, Antonidus etc.
I play it in a control dragon shaman build which isn't particularly competetive (although it holds it's own against everything but Quest Shaman) but is extremely fun to play.
One thing to bear in mind is that if you roll flobbidinous floop it will then stay locked as that card, i.e. it becomes flobbidinous and you play a twilight drake. Next turn you can play it as a 3/4 twilight drake. Useful if you have shudderwock but think it is probably a bug
I got it in my initial legendary pack opening and the simple fact is that it is playable, but not great (and something you should NEVER craft). You can highroll it hard, but it is far too unreliable to be truly good and many of the legendaries you hit are most useful in combo (e.g. I got a Prophet Velen once, which makes shaman burn cards kinda nuts), but the constant shifting makes it impossible to save a minion until you actually need it. If you could freeze it like in battlegrounds, it would be pretty nuts, but this isn't a mechanic in the game.