Having just played my first free game of arena I absolutely can't see any fun it that whatsoever. You might as well be allocated 30 random cards because the deck building is little more than that. Seeing as you have no idea what you are going to be offered further down the line and at what mana cost the deck has zero synergy (other than by pure chance of you being offered decent cards) and as a result of having to just take random minions, any success you have with the deck is purely down to what order you draw the cards in (i.e. it will never have any consistency from game to game.)
My deck had almost no duplicated minions ( I was offered very few duplicates) and is just rubbish. Zero synergy, useless minions, few spells that I would want, absolute rubbish frankly. It would make much better sense for the cards to be offered to you in progressively increasing mana cost brackets rather than the pick-and-mix way they are now.
It's a shame because that has pretty much killed off a lot of the enthusiasm I had for the game seeing as I can now see that getting better cards (once boosters become inefficient) are locked behind such a ridiculous mode.
I would absolutely begrudge having to pay money to play that mode as it would become extremely costly seeing as it's never going to be self-supporting from a cost basis. You might get a lucky run one time and get a nice card and some money for another run or two, but the next five runs could easily be 0-3 with how completely random it is.
That whole mode strikes me as a massive dead end to progress. I kind of regret having spent money on boosters now