You could try typing in "{product} review" into Youtube search to get video reviews of everything, where {product} is each TGC product.
I have a passing familiarity with DB, DBPro, that short-lived C++ library which was eventually merged with AppGameKit as AppGameKit Tier 2, AGK1, and FPS Creator. I'm currently going in-depth with AGK2 Tier 1, and I think I can sum it up as "the greatest BASIC ever". As for GameGuru and other TGC products, my current main computer doesn't have a graphics card (long story), so I haven't tried the other recent TGC products except to see that "Yep, that's an atrocious framerate. Time to uninstall.". I own GameGuru, I just can't really run it on my current hardware, so I have no opinion either way.
AGK Tier 1 is a fully-featured programming language and not just a scripting language. By definition, that makes it more educational than GML (used in Game Maker Studio), and gives you a lot more control over the overall structure of the program, but also not quite as straightforward as GML. I would definitely recommend GM:S2 over AGK2 for game jams and absolute beginners, but AGK2 seems much better as a professional tool. AGK2 doesn't obscure what's going on "under the hood" nearly as much as GM:S2 does, and that's either a huge benefit or a huge drawback depending on your needs.
