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|New Rating system (RIP Greven)|
Power | Flexibility | Synergy
3.0 - Total Bomb | 3.0 - Effectively Colorless | 3.0 - Best with others
2.0 - Play this card if you can | 2.0 - Splashable, 3 color | 2.0 - Better with some decks
1.0 - Filler | 1.0 - Playable in 2 color without problem | 1.0 - No special interactions
0.0 - Unplayable | 0.0- Needs 10+ colored sources | 0.0 - Sideboard

This is a very strange card. It's basically 2 1/1 fliers for 6 mana that doesn't cost you a card, but does have a small cost to your mana base.
I will usually run 1 of these unless I have very heavy 2 color commitments (or god forbid 3 colors!), but it's hard to run 2. Another option is to run it as an 18th land which is reasonable if you have a few 6s, in that sense it's kinda like meteorite. Of course, it doesn't technically ramp you but it doesn't cost 5 either!

1.5/2.0/1.5, but I don't usually pick it super highly. Note that if you have this AND Rogue's passage you are taking a pretty big risk in most 2 color limited decks.

I think I've been underrating this one. It's a fine playable. I've just seen Neal Oliver discuss a RW aggro deck that played eighteen lands to accommodate two of these and still have enough colored sources for his deck.

I posted in the comments of that article. I think that the 2nd Foundry is a reasonable choice given that he had literally no reasonable W or R playables for a 23rd card. But I would much rather another 2/2 or even 3 drop or maybe even combat trick (I think he only had 14 creatures + 2 foundries)

EDIT: I think he had Heavy Infantry and the 3/3 flier for 6 as options. Maybe a Volcanic Rambler? Stuff that was worse than Foundry.