I'd second the "Good, fast, cheap - pick two" sentiment. Before I started brewing, I burned through numerous cheap thermometers smoking meat.
About 10 years ago, I got a Thermopen. Great for an instant temp check on meat or a mash temp. The only problem is you have to pull the lid off the kettle and that lets heat out. I mash in my oven and I wanted to be able to just glance over at it and see how its doing. About 3 years ago I got the Chef alarm and drilled a hole in the lid to my kettle. That way I can drop a probe through the top, wrap it in a blanket, set it in the oven and still know what's going on. A side benefit is you can set a high temp alarm and drop the probe into your mash water with the included clip and go about your business and not have to worry about overshooting your mash water temps. And then last year, more for BBQ than brewing but I've been using it for both, I got the SMOKE which has a wireless receiver so I can monitor the temperatures and don't even have to be in the kitchen to know what is going on.
So 3 thermometers from Thermoworks and couldn't be happier. No problems what so ever. I have had to change the batteries on the Thermopen once but its been like ten years. Are they necessary? No, but its nice having them. I actually did a double batch on Monday and I used the Chef alarm to monitor one and the Smoke on the other one. If I had to pick just one, though, I'd go with the SMOKE, but then again that's cuz I do a lot of BBQ.