I've used quite a few split keyboards already, from the ergodox ez to the dygma defy but I've always wanted a split keyboard that has a mouse solution.
The Keyball44 is the first keyboard I've tried that answers this problem.
I ordered my Keyball44 fully assembled, I have little to no experience soldering something successfully so I am unable to provide any insight here. All I can say is that the keyboard seems sturdy.
The Good: This keyboard is great, the trackball is very useful. Aside from preventing the need for me to reach for my mouse, the trackball allows me to scroll both vertically and horizontally when in layer 3. This is extremely useful and I feel this can't be understated. Being able to scroll vertically and horizontally while keeping my hands on the keyboard adds a level of convenience that I almost can't live without.
I thought customizing this keyboard was going to be somewhat bothersome, as you will have to clone a forked branch repository off of QMK that is custom developed to handle the Sea Picro controller that HolyKeeb uses, however I found that I was still able to flash my keyboard by just using the web app remap-keys. The web app has some limitations such as a max of 3 layers but I found I was able map all the keys I needed.
Both halves of the keyboard can be flashed to have separate layouts depending on which side you plug the usb c cable into.
The Bad: The 40% layout is kind of limiting, I do wish there was a version with more keys, specifically for numbers. I've gotten used to the current layout somewhat however I would say that it's best suited for productivity. Gaming on this keyboard will be too difficult especially if you rely on number keys.
Switching to the keyball44 was difficult for me even having used several split programmable keyboards before. I had to rearrange my space, backspace, enter and tab keys as I now have less thumb keys than I did previously.
Currently, it doesn't look like a lot of people use this layout yet outside of japanese customers who have purchased this keyboard from Shirogane Labs. The few videos I've found on youtube are all in Japanese. If you do purchase this you should understand that you are sort of a pioneer in this space until there is more consumer adoption for a keyboard like this.
There is a discord community specifically for HolyKeebs however, so feel free to pop in if you have a question before or after purchasing your keyboard.
Conclusion: If you are okay with some tinkering and have always wanted a convenient keyboard trackball solution then I think you will enjoy this keyboard. You will have to provide your own keycaps and trackball but if you're someone who's already gone down the split keyboard rabbit hole like me then that shouldn't be too much of an issue. I definitely think keyboards like this are closer to the end game solution. I will definitely be buying another keyboard from HolyKeebs so I don't have to carry this one to and from the office!