Have a Garmin 500 with HR/Speed/Cadence. I mostly use it for tracking rides but also use it for training. I have set times I must finish certain routes to be sure I'm working hard enough, etc. Recently I discovered the 'Heart Rate Alert' function. I have it set to beep when I go into Z5, which for me is 191bpm. This is helpful for knowing when to back off on group rides. I'll probably have to turn it off when doing intervals.

I really enjoy the telemetry. It's helpful for me to see what my cadence/HR are doing on certain parts of my rides and correlating that to perceived effort. Most of this is done post-ride. I do a little screen watching while riding solo but in a group we're usually going hard enough that I can't be bothered to look down.