I have the Honda one for my Gen1, which probably won't work in the Gen3. That particular one fits in one of the block core plug holes and is relatively easy to install, but it is kinda underwhelming in terms of temperature rise. It does make the car warm up a bit faster on cold mornings.
Unless you live with very cold winters, I personally don't feel they are worth the trouble to install them. You engine is much quicker to warm itself up than any block heater. If you are trying to improve MPG from cold start, just don't reset your MPG gauge until the engine is warm
I'm on the Canadian Prairies and all our vehicles come with the block heater installed... -40 degrees C, its needed. I always joked with Americans that I had an electric car (when they saw they plug coming out the front). It really does make less wear tear on the engine for very, very cold starts.