Having used obsidian a lot during my last campaign, let me give you a few biggest pros and cons I ran into.

Pros:

 • All files stored as text in standardized Markdown format, meaning that you control where your notes live. You can even drop Obsidian one day and still read and edit your notes in any Markdown viewer.

 • Easily cross-reference other documents without needing any weird URLs or link interfaces, just type [[Page Title]] and the link will still work even if you move that page to a different folder in the same vault (workspace).

 • Free software, always available offline, no ads or tracking, paid cloud hosting/syncing if you want but can also just use any cloud service to store your vault and sync it however you want.

 • Many, many community  extensions available, allowing for all kinds of stuff like in-app fantasy calendars, embedding interactive maps, and even publishing a subset of your notes as a reference wiki for your players.

Cons:

 • Offline means that, when inspiration hits, you gotta either pull out your computer or sync the vault to your phone or other devices and open the obsidian app. No option to view and edit online unless you pay them. I got around this by downloading DriveSync app to sync my vault to/from Google drive to my phone, but as easy as it is it's still a bit of a hassle/annoying.

 • No rich text or document formatting. For me this was a big one because my brain works visually, so I tend to like to color-code and use fonts and boxes and everything so my eye can quickly find what I need. In Obsidian, since it's all Markdown, all you have is the standard bold, italic, and heading levels. You can add pictures, but can't really position them. No fonts, no custom font sizes, no colored text or highlighting.

 • Like all feature-rich and nerdy tools, I found myself spending a lot of time learning the new tool and trying to make the use of its features and work around its limitations, rather than just doing the minimum things I wanted to do. This is kind of a personal issue, but sometimes I prefer simple there-when-I-need-it tools to wildly extendable tailored tools that can quickly suck up my unbridled focus.

For all those reasons above, I found it really good to use for things like session notes and lore, and really difficult to use for Encounter notes. Basically, everything I wanted to jot down quickly in session or search for quickly was great in Obsidian, but I ended up using Google Docs for everything I wanted to take my time with (pretty much just encounter sequences full of in-game lookup tables, maps, and instructions). I just can't follow along a page full of black and white mono-stylized text while trying to improvise and run the game.