I've known about instant read thermometers for a long time, but never really paid a whole lot of attention to them - just another cool gadget that'll sit in the drawer with the rest, I thought. Well, I got my first instant read thermometer a few months ago and my grilling hasn't been the same since - I never realized how useful these are.
I've always used a traditional meat thermometer - either a standard (non-electronic) one, or electronic but slow. Often, I'd end up with meat that was over or under done from what I expected. The difference between getting the temperature in 30 seconds versus 2 really helps with getting a consistent cook and nailing doneness. Even when smoking with a probe in, it's useful for spot checking the rest of whatever I'm cooking.
I bought a Thermoworks Thermopop, but there are a number of other models and manufactures making the same sort of thing. Worth checking out if you are an amateur like me and cook by temperature, not by "feel".