Well, yes. Buying...well that depends.
SoI was mechanics-light compared ot most of the other Tribunal books. That being said, a lot of the innovative things it did have been taken on board and used by later books, most notably GotF and HoH:MC. There's still a lot of useful stuff in there, in story and setting terms, but if you have a choice between it and one of the books in the new edition, and there's nothing in your saga theme that prods you one way or the other, then perhaps the newer book is the better way to go in terms of purchasing.
That being said, if you want crochety old covenants, a culture that tells younglings they are not important until their elders tell them so, and politcis played at the level where war is unthinkable because the damage is incalulable, then maybe it works for you. I still love most of it, but then again, I wrote it, so I'm biased. The only thing I'd change is the Criamon, where I didn't take enough risks the first time around.
Oh...and I'd tweak the Jerbitons. They are a bit too...well everything for everyone? I'd give a bit more...oh, I don't know, but they need a bit more of something.
Those things aside, I like how the redcaps turned out. I like the chapter houses, which weren't my idea (they are in "Deadly Legacy") but I like the way they were used. I think the cats are good. I like the Lightning Lineage. and the Idea of True Enemies for Tytalus magi. I like the idea that the Order might have a secretly-designated war leader, who is waiting, hoping he doesn't need to use his powers. I like that the faeries aren't sex-and-song-in-the-forest. I like the devil's bridge ideas...
So, generally, I still like it, and think its worth a read. Buying's up to what suits you, really. If you can only buy one tribunal book, then buy GotF, because it will give you more play, IMO.