I have a glass "beer and wine" termometer that I picked up at the local wine shop and I think it's accurate but it's painfully slow to come to temp. It floats so I leave it in my mash water as it's heating but when I adjust my mash temp, it takes forever. I pulled it out of the 170 degree mash water, let it cool to about 150 and put it back in and it took about 20 seconds to get to back to temp. It drops fast, rises slow.
So, when I added my mash water, I was a touch hot and so I added some cold and it dropped to 140, so I added a bit more hot to get it back up and it may have eventually got there. I gave up and used the kitched thermometer which is celsius and got it to 67C which is about 153F. This one is very small and hard to read though. Also, I have no idea how accurate it is although it did correspond with the slow one in the mash water.
Just a bit of a pain to fine tune temps without loosing too much heat when it's so slow.
So, what's the best, quickest, best value thermometer out there for brewing?