Does telemetry work well? What are your experiences with it?

Temp sensor can be useful when setting a car up correctly

Voltage is useful if running a nitro

Cheers

Are lap times or car speed useful if racing? Or just extra that you don't really need?

Tracks usually have their own timing systems, no reason to have your own unless you are messing about by yourself

Cheers

Ok thanks a lot for all your help man!

It works really well, and I love it.

I like knowing how my motor and esc are doing temp wise. Each has an alarm in case they are getting too warm for my liking. I do monitor the motor rpm just to see what it turns during a race.

It's all a waist of time and money.

Once you apply yourself to properly applying it, telemetry can be a very good thing. I have been using it for years to monitor engine temps and battery voltages on my 1/8 onroad car. It has saved me from putting a car on the track with dead RX/TX batteries, needlessly roasting motors, needlessly roasting steering servos, and other scenarios.

Yes

What's a good price for a telemetry receiver for a dx3r pro?

You use them more for practice and setup because you won't be looking at the data during a race. Voltage and motor temps with a signal when they go out of range can be very handy during practice.

Gotta love telemetry. If you have the right RC transmitter it will even warn you when your lipo's are at 20% capacity. That's when you should stop to get maximum life span out of your lipos.

and for helicopter pilots.... well.. you don't want your heli to crash cause of an empty lipo .

When you have advanced equipment rotorspeed and motor temperature would be good to monitor when flying heli's.

Check out this RC transmitter, it does support telemetry. I use the dx8 myself and loving it.
http://www.rc-heli-review.com/rc-tra.../spektrum-dx7s

Good Luck

Yes. Temp sensor for real time temperature readings. RPM to help you tune or see if you are driving hard enough. (e.g. if I can hit higher rpms at the end of 100 feet).

And lap timers if the track AMB/computer is not turned on...