I can only presume that people who think Spellslinger is garbage in arena are just trying to stop mages from picking up another Spider Tank because God knows they don't need any help in arena.
I think it's pretty weak in arena because games are usually slower and the randomness of the spells can affect the game much more heavily. It's also hard to draft a deck that would offer you synergy with spells also.
The 3/4 body on 3 is great, and if you look the text as equal for both players it's a wash. It would really come down to what it's offered with, I would pick it over Razorfen Hunter. I would not pick it over Spider tank.
To exploit the cards value you have to be low on cards, heavy on tempo, and loaded with spell synergy. If you draft the right deck this card could be killer.
I fail to see how this wouldn't blow up in the mages face. There so many spells in the game that odds are it will be blingtron all over again. Not worth the risk since it will likely just backfire in one way or another. Nice stats though.
You build the deck to counter the risk. Your spell costs 1 less(apprentice), your spell does 2 extra random damage(flamewaker), your spell grants +1 attack(mana worm), on top of whatever other spell synergy cards mage gets.
In an even more perfect world, you would have a tempo lead. You could play your spell and kill your opponent before he ever gets to cast his spell.
This card has a lot more RNG, but another good example is Mechanical Yeti. Where it's played in decks that take advantage of the spare part with Antonidas.