Initial thoughts after a few days, and whether I regret not buying the SL2-S instead:
Size and weight were always a concern. From the first time I held the S1 in 2019, it was apparent just how heavy and large it was. My biggest camera is the Canon 5D Mark II, which still weighs about 810g to the S1's 1024g. It's noticeable, and definitely a big change compared with the M9 that is regularly in my bag. But I bought the S1 as an video-capable alternative to the M9, and I'm trying to convince myself that by carrying the S1, I don't have to carry anything else that shoots video. It's heavy but not more than the M9 and Panasonic G9 in the same bag, which I sometimes do.
Given that I used to carry the 5D Mark II with 24-105 and 35L, I can get used to the S1 plus a couple of M mount or small Sigma Contemporary lenses. Give me another four years and mayyyybe I'll get a 785g SL3-S when the prices are better, unless Panasonic make another flagship pro body but of that weight.
S1 - Melburn by
Archiver, on Flickr
Handling and haptics; the S1 was designed as Panasonic's premium flagship professional body for L mount, and they pulled out all the stops with build quality and handling. The buttons feel positive and smooth, the grip molds itself to the hand, and there is a plethora of custom buttons all over the camera. The camera feels so solid and reliable, and the grip means you just want to shoot with it.
S1 - He Points The Way by
Archiver, on Flickr
Manual focus: As the S1 was bought with the intention of a Leica M9 alternative, I wanted to be able to use M mount lenses with minimal negative impact. My most used M mount lens is the Distagon 35 (thank you to Michael Bialecki and other RFFer's for the recommendation) and it works a treat on the S1 (and S5). Super sharp, lots of pop, and great colours. The Summicron 50 also works great on the S1.
The EVF is huge and clear, and the punch-in is considerably better than the S5. With the diopter correctly adjusted and focus peaking on, sometimes I don't even need to punch in. Especially with a fast lens, focus can be accurate and snappy as I continue to learn to dial it in and recognize when it's in focus. Obviously, it's very different from the rangefinder split-image method, which I still think is the fastest manual focus system, but the S1's huge EVF makes a lot of difference.
S1 - Dumpling by
Archiver, on Flickr
Do I regret not buying the SL2-S? Not really. The S1 has a tilting screen, which has been very useful already for waist and chest level shooting, or shooting overhead. The SL2S has a fixed screen, making those actions much more difficult, if not impossible. My brief time handling them side by side showed me that the S1, despite being heavier and chunkier, sits much better in the hand. Do I like the IDEA of the SL2S? Of course. But the practicality of the S1 is right up there. For future updates, I'll start a new thread here in the SL forum.