What is the crowd's opinion of Source SDK?

From 3.5 billion comments

Free. You also have to have the source SDK, which is also free if you own a valve game.. I did not need to download the SDK, as I have half life 2, and tf2 already.. things just worked right out of the box with BM.

source SDK

Black Mesa

Nobody has mentioned Source SDK, but it's an obvious choice if you'll be distributing via Steam. It's cross platform (Win + Mac) and is obviously a very high-quality engine. It's supposed to be very easy to work with as well.

I don't know anything about licensing costs because those are only available under NDA. There are a few Source-based games from small developers that sell on Steam such as Garry's Mod, so there must be some kind of indie-friendly licensing, perhaps similar to UDK or Unity.

That's actually 100% incorrect. Valve is completely open with Source, I think all you need is to own a Source game, including TF2, meaning it is essentially free. Source SDK is open and free for the community to use.

Source(Pun slightly intended): http://source.valvesoftware.com/sourcesdk.php