Customizable card games are a favorite game genre for me, and have been ever since I first discovered them way back in my college days. I must admit, though, that I'm not very good at the deck construction part, at least not for competitive play. So I tend to gravitate toward limited formats, with a restricted card pool used in a short series of games. Hearthstone's Arena Mode is one of my favorites.
I won't go into a discussion of how to do well in Arena, both because I'm no more than an average player, and because there's plenty of other guides out there. (Like this, or this.) But I will mention a few things that I especially like about how Hearthstone has implement the limited format with Arena Mode: reasonable cost, scheduling, and the shifting format.
The cost to enter the Arena is 150 gold (or $1.99, but I've only ever used real money once, and that was really just to show support for the game). For that price, you're guaranteed a card pack at the end of the run, which would cost 100 gold if you bought it outright at the store. If you win 3-4 games, you should also get 50+ gold (and maybe some other things), so you'll basically break even. If you get on a hot streak and win 7+ games, you'll get back 150+ gold, enough for another Arena entry. That last is rare, but the break-even point is pretty common. Doing Hearthstone's daily quests can get the 150 gold entry fee every 3-4 days, and games played in the Arena count toward those quest goals. All this adds up to the ability to play the Arena very often even on a free-to-play account.
I appreciate that the Arena is set up so you can complete your run on your own schedule. Once you start a game, you have to finish it like any other game. Other than that, though, you're totally on your own time. There's no timer between rounds, so if you want to leave for a few minutes or hours or even days before starting your next game, that's no problem. Even the initial selection of class and cards has no timer - you can stop in the middle and come back later. Much easier than the traditional draft or sealed formats that require all the participants together for a limited timeframe.
As new cards are released from expansions or the single-player adventures, those cards make their way into the Arena choices. This both keeps the Arena format interesting as new cards are added, and gives players a chance to see the new cards even if the player can't afford to buy them right away. I like how this shifts the balance in the Arena as the game adds new content.
If not for the Arena mode, I'm sure I'd play a lot less Hearthstone. As it is, I usually get in one or two runs each week. It's one of the better implementations of a TCG limited format that I've seen.