This is true. Accurite is a particularly good one. But, the first one I got was this little thing from Lowe's that reads temp and humidity (no proble) and comes with a suction cup. I have a few accurites now but I still always find a place for it. It was only $7 and it beats having nothing at all by a long shot!
One bit of advice with the accurites - the first one I got I broke inside of 10 minutes. The probe wire is fairly delicate and I was trying to run it around the cage lid and it got pinched. Tricky thing is, when this happened, it just continued to read the same temperature forever. I've purchased several more since then but I make sure to be careful with the wire.