Having seen the success this has had in various Mardu, Rakdos, Boros, and even Gruul standard and historic builds, I am going to go ahead and test this one against the consensus here that this is a "win more" card. I would argue this is the best of the legendary artifacts cycle even outperforming The Great Henge and is probably the best cubeable "combat trick" we are going to get for a long time.
I don't believe this is leagues worse the Heirloom Blade, in fact I'd say that is about the tier level of this equipment. If anything this is slightly better. Still, this is all theorycrafting for now, let's revisit this once we've tested it in cube for sometime. We don't have anything quite like this in cube so it's tricky to evaluate.
This is leagues better than Godsend, but I agree that is a constraint on its general utility, but I also think it will help give Red a tool that will entice players into the color--which ,at least for me, is sorely needed as Red tends to be the least popular color in my experience.
I think this one merits testing as opposed to just theorycrafting it out of your cube. I wouldn't be surprised if we see a whole bunch of "well, that was unexpected" posts come out of this card. Flash + free equip + double strike/pump/evasion is a stupidly powerful combination already and then they threw in a cost reduction ability too? Hell, you could even Tinker into this for some serious LOLS.
Even at 4 mana this turns an unblocked 2/1 into 6 damage while they blocked what appeared to be the relevant threat. While, I do think this can be a win more card, I think it can also just be a win out of nowhere card and this will make for some awesome stories.
Legendary Artifact - Equipment (M)
~ costs 1 less for each attacking creature you control.
When ~ ETB attach it to target creature you control.
Equipped creature gets +1/+1, has double-strike, and trample.
This might be the closest thing we get to a cubeable combat trick and I want to test it....