There are positives and negatives about it. I went all in with a Legendary bundle* and Master Tier subscription so I have access to everything. That skews things a little in favour of DDB when considering the value of ToA on it, because I'm not locked out of any content and can use it as a total resource rather than a limited resource. However, every monster from Volo's that is used in ToA, is included as part of the purchase of ToA, so that's somewhat mitigated.
There are some issues with it. Some teething problems with links but they're being fixed up. My main gripe with the DDB format is how the menus work. There is an index page where you can go to individual chapters. On each of the chapter pages, however, the only way to access that index page in order to access other chapters, is to go back to it. That requires having it open as it's own tab at all times which is interminably frustrating in terms of navigating the document. Having said that, it's also available in the search bar so if you know what you're looking for, it's only a search away.
Otherwise, I find it fantastic. The maps alone are almost worth as much as the price of the book, with player and DM versions for almost every one of them. Plus artwork and individual monster entries. It's an entire book replacement for use at the table. As a DM's resource, I find it amazing and something I've been wanting for over 20 years.
*I did this because whatever issues I feel DDB currently has, I'm confident that the team will fix them and that they'll consistently and constantly work to make DDB better.