1. Not many players (few hundred!)
2. Not very active (maybe 60-70 games at busy hours and 5 at dead-hour

)
Is it worth it? Oh boy yes. But you need a good joystick and learning curve to pick it up. Descent 3 is the most diverse game out there and the only one of it's kind. And the adrenaline rush in some multiplayer such as CTF is the best of ANYTHING
If you would like some multiplayer screenshots I can provide..
You can find D3 probably at
www.amazon.com and
www.ebay.com