I'm playing it since a few days now and must say that this game is at the moment extremely bare bones, but has incredible potential.
The biggest flaw by far is their focus on only 1vs1 duels.
While some form of multiplayer (2 headed giant, at least) is planned for later on, the current open beta doesn't even have a friend list.
That's right, an accessible MTG game for an online audience and you can't even play against your friends. Friend list is the next feature they're working on, but I think you can say that they really got their priorities all wrong.
You also get showered with free decks as you play and fulfill your daily/weekly quests.
Problem is, about half of them are just terrible decks - while the other half are surprisingly good. I don't know why they have such a variance in free deck quality, what's the point of that? In some decks, almost every card synergizes with almost every other card while in other decks, there's barely any synergy at all. A bit of an oddity.
I think the prices for attending sealed events or drafts are bit too high, but at least not fully unacceptable. It does suck that you can attend some events only for real money and not the "earnable" money, though.
One thing I find commendable is how they deal with duplicates. Having more than four copies of a card doesn't make sense, so when you get more from a booster, what happens is that they go to the "vault". As the vault fills, you are awarded wildcards. There are wildcards of every rarity and you can permanently exchange a wildcard for ANY card of the same rarity - really a good idea to not force people to buy endless boosters just to get a certain card. There's no trading in MtG Arena (it was never meant to replace MtG Online).
The meat of the game, though, is without a doubt the best MtG experience I ever had on the computer. The flow is fantastic, you have all the control you need and want, they are offering a great overview over castables (even if in graveyard, for example), you always know in which phase you are, etc. There is fantastic potential here and I hope they won't drop the game like they did all those "yearly" Duels of the Planeswalkers games.
At the moment, I'm playing my daily stuff and basically wait for the game to receive some more game modes and finally some multiplayer to actually play WITH friends and not just against them - though, to play against them would be at least something and as I said is coming soon.
Now if only I knew what to put together next - mono red, izzet , boros angels all look really interesting.
What I do is make a deck in which I put all the cards I'd find interesting to build a deck around. Kind of an "ideas" deck. Makes it easier than just looking at all rares/mythics at once.