This version that I have been using is just the public preview, things will change from this point but it is a good indication of what the IDE is going to look and feel like. At the moment you have to install Fleet with Jet Brains Toolbox making it less of a lightweight IDE but that will change. My main work machine is a silicon Mac, some of the issues that I ran into could just be configuration issues with the mac version as I have seen some other posts about M1 and M2 issues.
The Good
I like Fleet as a standalone IDE, It's clean and fairly minimalistic. The startup time and responsiveness were on par with VS Code. Fleet’s smart mode also worked decently well for javascript, python, and typescript but those were the only languages I could get working. The side panels are nice, git versioning works well and so does the docker window. One thing I wish I had gotten to test was the collaboration feature but apart from that, Fleet functions as a very standard IDE.
The Bad
Besides the nuanced changes that I had to get used to switching from VS Code to Fleet, there were a lot of annoying things that Fleet does. Data sharing is automatically turned on and so is autosave. I don't want my files to behave as if I were working on them in google docs. Additionally, for a lot of the settings in Fleet, you have to know the setting and directly change it in the settings.json file. While this makes it easy to quickly look at all of your settings variables, it's annoying when you have to go searching their docs for a niche setting since it isn't on the settings page. Many of the languages also weren't working with the smart mode at all for me, below is a spreadsheet of the languages that are supposed to be currently compatible. I found three of them were able to work.
Final Thoughts
Fleet is a promising editor which is only in a preview version now. It is far from a complete product. The current iteration of smart mode can't be used in professional projects as even when I did get it to work on mac, it still had trouble recognizing certain language features and imports. I don’t think Fleet will ever really be able to compete with a 100% free tool with extensions like Sublime or VS Code. Jet Brains has announced that plugins will be coming soon but it will be difficult to rival the ecosystems that currently exist. That being said, if you are already using all the other Jet Brains products I can see why you would add this IDE to your daily workflow in the future. The smart mode is nice and I am sure that they will improve it but it's personally not enough to convince me to switch over from VS Code.