MPSA42

[saturated]

I'm the proud owner of some now

Hopefully a situation arises where I need one or two 

I thought the specs were somewhat intriguing 

And the price $$ is nice 

[tonyharker]

What's special about those?

[Kaz Kylheku]

They are high voltage: collector-emitter voltage can go to 200V. 300V for the MPSA43.

They are useful for low-current, high(er) voltage amplification and switching.

[J M Fahey]

Everything is a compromise.
They get higher Vce,  200V, so useful in high power amplifiers which work with high voltage rails but pay for that with quite reduced current gain Hfe , so I wouldn´t build a preamp or pedal with them.
They are not your typical general purpose transistor.

[joecool85]

Interestingly, I had some of these kicking around (no idea how I came to have them, but I've had them for YEARS) and am using them for an RGB controller.  It's only running 9v, but I had no other use for them and they work well.