I borrowed a friend's J 2 for about two weeks to run in my horn-based system (99 db/watt efficient). I liked what I heard--dynamic, smooth, clear and detailed sound, but I would not mistake it for the tube amplification that it replaced. It still sounded distinctly solid state, meaning it had a slightly artificially hard edge to the attack of notes. While it did not render the sound as brittle as most other solid state amps, it was still unlike the low-powered tube amps that I prefer (there are plenty of tube amps that sound more brittle than the J-2). Overall, I liked the sound a lot, and could easily live with the J-2; I happen to prefer my tube amps, but, all of them cost many multiples of the J-2.
I also liked the fact that the J-2 did not have much in the way of negative operational issues. I heard no turn-on or turn-off noises with my high efficiency speakers and they ran noiselessly. But, they run a little hot, so I would not leave them on all of the time. They do take more time warming up than is the case with tube amps so they don't sound their best for at least 20 minutes or so.
I also heard a home-built version of the First Watt SIT amp, and that is also a winner. I like this brand of amps a lot, more so than some of the Pass Lab high powered amps that sound a little bit lifeless to me at low power (common problem with almost all high-powered solid state amps).