I've had the Korg C1 for about a year now, and overall, I'm quite impressed with how it simulates the feel and sound of an acoustic piano.
The key action is one of its strong points. Korg uses the RH3 keybed, which is graded and weighted to provide a realistic feel. While it might not perfectly match the nuance of a high-end acoustic piano, it's remarkably close for a digital option, offering good dynamic response that should suit your focus on classical and jazz.
Regarding sound samples, the C1 excels with its rich acoustic piano voices, with samples taken from famous concert grand pianos. It's pretty convincing, especially when played through good headphones or quality speakers. The nuances in soft passages and the ability to cut through with more aggressive playing are both well-captured.
In terms of build quality, the C1 feels robust. It's a nice piece of furniture as well as an instrument and feels like it could withstand years of regular use without issues.
Some useful features include the dual headphone outputs and the recording function, which can be helpful for practice. However, it doesn't have much in the way of connectivity options, like Bluetooth, which could be a downside if that's important to you.
If I had to change one thing, it would probably be expanding the sound library. While the piano sounds are excellent, a bit more diversity could benefit those wanting to experiment with different musical styles.
I would also suggest trying it out in a showroom if possible, just to ensure it matches your personal touch and feel preferences. Hope this helps! What are you currently using, and how does it compare?