I think after this review a lot more SPCR users may be switching to PicoPSU units.
A couple of downsides which put me off them:
1) Good luck finding a 12V AC/DC brick >100W without a fan.
2) Good luck finding a 12V AC/DC brick >100W for less than Â£40 here in the UK.
3) a picoPSU + AC/DC brick will run about Â£70; an S12-330 is ~Â£40.
So the real niche for these things is for people with ultra-compact mini-ITX or micro-ATX setups with low-powered CPU's. If size isn't an issue the S12 is cheaper and more convenient.
DC power supplies of much larger capacities are availble and widely used for radio work. These are not cheap but they are also of much higher quality than typical brick power supplies. With much lower levels of ripple and much tighter voltage regulation over a very wide range of power levels. Most have a default voltage of 13.8 V DC but most can be adjusted over a range of voltages. Some internally and some with an external control knob. I have seen models over 500 watts that do not have a fan and most under 300 watts do not have fans.
For example the Astron VS-35M can be adjusted for any DC voltage between 2 and 15 volts. Has built in voltage and amperage meters and will handle 25 amps continuous (300 watts) and 35 amps with a 50% duty cycle (420 watts). Ripple is 5mV peak to peak and voltage regulation is +- 0.05 volts. You can get one of these for <$200 if you shop around. The same basic power supply with out the meters and that would require you to open the unit to adjust the voltage can be had for <$160 (RS-35A).
Need more power? How about the RS-50A/VS-50M at 37 amps continuous (444 watts). Still not enough? How about the RS-70A/VS-70M at 57 amps continuous (684 watts). None of these have a fan. They are however large and heavy (25 to 60 pounds) and expensive ($150 to $300 depending on the model).
All of these can be had with an optional built in battery back up system. All you have to do is add the battery.
The real question is not the avilablility of power supplies capable of powering very high performance PCs rather it is the question of the ability of DC to DC converters like the PicoPSU to handle the other loads in such a PC.