I use a digital meat thermometer from the kitchen aisle in the grocery store. I figured I have to go to the tank to read the temp anyway (and I have to get really close since I can't read anything that tiny), so why keep it in the tank? It's one less thing to try to hide.
The thermometer I have takes about 10-15 seconds to come up to full temp. I've tested it against some hyper-sensitive equipment from work and it's accurate to w/in .5 degrees all the time. It cost about $15. Not the cheapest solution necessarily, but much more convenient. It's great for setting the water temp for water changes too. Just hold it under the tap until it's right, and then straight into the tank.
Hope this helps.