Using it since many years on many Pixels and loving it.
Main pros: zero bloat, efficient, highly secure and highly private (about as private and secure as it can get on any smartphone), and it's an Android without any of Android's typical weaknesses (privacy issues, bloat, etc.). You get to utilize the advantages of Google (its security) and completely avoid the disadvantages (its many privacy issues). You get to use all the advantages of an Android mobile OS while completely avoiding all of its disadvantages. It's like getting your cake and eating it too. You're much better off in terms of security and privacy than any other smartphone user. This isn't even marketing lies, e.g. according to leaked documents, Cellebrite for example can't crack GrapheneOS on Pixels at all. They can crack almost any other smartphone if they have physical access to it. Most smartphones are really easy for them to crack. iPhones may pose some trouble depending on model/OS. And Graphene is the literal brick wall. And even on top of that it has tons of great security features, like auto-reboot after X hours of inactivity, charge-only-mode for USB-C when locked, distress/duress PIN entry to immediately wipe the phone, many things like that. On the privacy side it's looking great as well: Some folks have analyzed Graphene's network traffic and there's zero privacy issues from the OS or its built-in apps. And the few connections it does make (for updates and so on) are all documented and work exactly like they documented them, and they only transmit the exact least amount of necessary data without anything beyond that (guess what - that's super rare). And on top of that there's even more great privacy features, some of which are invisible but well thought-out, for example any SUPL request goes through a Graphene proxy server first (configurable) which strips all personally-identifiable data from the request and then redirects it to your provider's SUPL server (which is most likely Google's SUPL server in the end). I'm seriously impressed by the quality of the GrapheneOS project. Maybe you don't realize how good and rare such things are nowadays. Also the documentation is very good and actually answers most of your questions and doesn't contain any marketing blurb. The social media feeds and forums are a great source of info as well. On top of all that it's even easy to install GrapheneOS.
Main cons: it's only available on Google Pixel phones, so if you truly despise Google and don't want to buy or use anything from them, it's not the right device/OS for you (or maybe buy it used?). However, the reason GrapheneOS is on Pixel is purely a technical one: Pixels do offer very high hardware based security already (probably the most, although iPhones have good hardware-based security as well. As is known, Apple tends to be produce good quality hardware, not quite so good software) as well as a very high degree of "platform neutrality", i.e. it's supported by Google to flash a different OS on it or use more advanced tools like adb without any sort of tinkering or unnecessary danger involved. Also you don't have to register to unlock your phone or anything, you only need to be online once to enable the OEM unlocking feature (I think this is because Google needs your IMEI to check whether the phone is carrier-locked (cannot ever be OEM unlocked) or can be unlocked, and they will immediately receive some device data including the IMEI as soon as you go online with the preinstalled Android OS once [of course they will receive some more device data than just the IMEI]), so it's best to not insert your SIM yet (and not do anything with the preinstalled OS) before you've installed GrapheneOS on your new Pixel. Do the OEM unlocking step on WiFi only, best on a public WiFi so Google has much less of a chance to identify you based on your IP or related data. Then install Graphene, then insert your SIM and start using your new phone. Other cons exist but they're rare or pretty much irrelevant in daily use. If you have to hear them, read an older post by me about some potential downsides: https://discuss.tchncs.de/post/19867254/12069767