I picked up the DDB version of Storm King's Thunder this week, in large part because its chapter on the North is a very useful resource to have on my computer when doing game prep (I also got Sword Coast Adventurer's Guide for the same reason). I had a look at the adventure itself, and also had it open during play. They don't have a lot of promo material for this, as you note, but then it's not really all that 'sexy'; what you get is a series of pages on the site, accessed via Compendium -> Adventures, and those pages have the full text and images from the book, presented like any other webpage with navigation tools, hyperlinks, and lots of scrolling. As it's a website, you can copy/paste the text freely, adjust the font size in your settings, and access it from any device with internet access. The images can also be downloaded, but they are undeniably more attractive in the book than when inserted into a webpage. That's kind of the extent of the adventure; the other stuff - monsters, magic items, whatever - also get unlocked if you go for the full book price, but you'll already have opinions on those parts of the website, I reckon.
Would I recommend buying adventures on the site? That's tough to answer. For reference, I own every 5e book, including adventures, and run my games at home with them all on a bookshelf next to my chair. I also
like books. Thus I find it hard to justify buying them all again in an electronic format. That said, the DDB versions are well done, easy to use, and - unlike a PDF or whatever - don't require special apps to open and restrict your ability to interact with the text and art. You can link to them in your notes files, and, if like me you run your games from an iPad, that's damn handy (e.g. "The players might decide to go to
Amphail; if they do, they'll meet..."). You should also consider that the adventures are much cheaper on DDB - about twenty quid rather than forty, for me.
Overall, I'd suggest that it will depend a lot on how you do your game prep and how you like to access information. I think that it's a solid alternative to the book, not least because it's cheaper, but I personally bought Tomb of Annihilation in physical form regardless. I would also say that it's a nice tool to have
alongside the physical book, but that it's harder to justify on those grounds, especially without an encounter builder pre-loaded or anything similar.
I hope that this helps, and sorry that I don't have a video to show you!
