I wrote a fairly lengthy review back when the Campaign Guide came out. I detest the new Realms, but I do go into some detail on what the book has eaten. It's here:
A Rant and a Review: Forgotten Realms Campaign Guide « Worlds in a Handful of Dice
Just to say that this is my favourite review of the book to date, because it's both informative and is highly entertaining to read. (Favourite part: "[Some NPC] trades child slaves for the serpentfolk living nearby. They’re stored in barrels, shipped down the river to Loudwater, and there Zark hires the PCs to unload the cargo. There are hundreds of ways to handle slave trade well, and none of them involve bringing outsiders in direct contact with the merchandise. Another spectacularly idiotic plotline, here.") It also contains absolutely enviable prose, and is informed by a breadth and depth of learning (that most detested qualities in negative D&D reviews) that I haven't seen matched in quite some time.
Chiming in with some personal experience, I've for instance used the FRCG
and the FRPG when prepping Skyfire Wastes, an early LFR try-out in and around Calimsham. I posted all relevant info from both books together, and still felt there wasn't really much (btw the info in "how to explain the Realms to your players" is much superior in FRPG to FRCG). So I guessit's fair to say that these books aren't source books as such (where 2E and 3E produced FR "source books") but
collections of hooks to inspire you. There's not nearly enough to make for compelling adventure scenarios unless 1. you like the original hook greatly (a purely subjective matter, on which my experience has been mostly negative so far) and 2. you've plenty of inspiration to draw on to elaborate these hooks. Now 1. and 2. are closely related, because the quality of a hook goes a long way to fire your imagination.
In terms of scale, I'd compare this to the Greyhawk Red Box with its mini-write-ups for individual regions. In terms of quality, there's nothing like making your own opinion. Check out the previews on the WotC website for the book and make up your mind. And I mean it. Print one of these, or several of them, out and then sit down at your table for 30 minutes and see if you can work with it. If you can, that's a good sign. If you find them flat, stay away.