Erm... wow, this is quite a bit better than Boros Signet. It's better than Ur-Golem's Eye, in fact, although many things are.

Hmmmm. This is now interesting. My first thought is that it's worse than Manalith (and a fortiori Darksteel Ingot / Spectral Searchlight / Coalition Relic). But actually that's not clearly true. To take the extreme case, this will let you splash Lightning Angel in a mono-black deck. (I did actually once have a nearly-mono-red deck - necessarily so because of Kiki-Jiki - but splashing a few cards such as Sky Hussar off 4 Azorius Chancery and 4 Azorius Signet. So this isn't as far-fetched an example as it might seem. This card would also have been great in that deck.) So I think the power level might be about right here.

It's risky, of course, because the Obelisk of Bant cycle was derided and pros generally didn't play them, preferring to play a more risky deck with higher-quality cards. You don't want your mana fixing to end up in that category. But I think this is a bit better than them.