I have downloaded a bunch of products already from the DMs Guild, and paid at least something for most of them, but only have had a chance to really examine a few yet. I mostly bought monster stats, but also some expanded rules on alchemy, poisons, etc. and a few player options PDFs like the Gunslinger. Here are my thoughts on a few of the ones I've bought, read, but not yet used in my games:
D&D Denizens: Goblins - 4 out of 5
D&D Denizens: Orcs - 4 out of 5
D&D Denizens: Drow and Driders- 4 out of 5
D&D Citizens: Dwarves- 4 out of 5
D&D Citizens: Elves- 4 out of 5
I'll review all of these together. They are all by Scott Holden, layout by Pauline Benney. I think the concept for these is great, at least for busy DMs like me who have little time to prepare and like using a variety of these basic monsters and NPCs in encounters. I read through the stat blocks and they seem fine at a glance, but I won't know if there are hidden problems or if the Challenge rating is accurate until I use them in play against my players in one of my games. I would tentatively give these five out of five stars for content (pending how they work in actual play), except for the layout. While the PDF looks nice and clean, the stat blocks are frustratingly difficult to read because the PDF has two columns and the stat blocks are not kept together so that they are often broken up between one column and the next or one page and the next, and it is easy to get confused in the middle of running a complex encounter and read the stats from the wrong goblin or orc, etc. in a combat. The layout requires I rate this four out of five stars, but I would still recommend these for DMs looking to spice up encounters with humanoids and demihumans which I think are among the most interesting encounters, along with humans.
Monster Mausoleum by Stacy Janssen and David Noonan - 5 out of 5 stars
This one was a great buy. It has a wide variety of classic undead from various books from prior editions, updated to Fifth edition with a good execution of their essential powers that made their original versions terrifying in play. The stats look well put together at reading. I still have the same caveat that I have to use them in play before I really know if the Challenge ratings are accurate but just reading each entry made me really want to run each of these in my campaign. And unlike the products I rated above, the stat blocks are well laid out so each combat stat block is on its own page and easily identified. This is exactly the kind of monster PDF I'd like to see more of in the DMs Guild and will fill the need for more monsters for my games in the months to come.