I do not like the in-sink hot water heaters. Before too long, they develop heavy scale on the inside which then flakes into your drink (as was mentioned previously). Not sure if there is supposed to be routine maintenance with these things but I ended up throwing it away. Before that happens, the water definitely tastes bad.
I have since gone with a desktop hot water heater which are pretty commonplace in asia. Best part is that they are easy to clean--just like you would a coffee maker...a chore I do about once ever 3 weeks. The water is really really hot and the only two downsides really is that it must be filled periodically (unlike the in-sink variety) and takes up some countertop space. In some households, there is no outlet under the sink so this desktop unit is a nice alternative.
Here is an example but the popular brands include Zojirushi, Panasonic, Sanyo, etc.:
http://www.amazon.com/exec/obidos/tg/de...E/qid%3D1129329659/104-0681907-0699112
Top uses (for me):
(1) French Pressed coffee -- the gold standard for preparing the most flavorful coffee! Amazon has some nice french presses too ($15-20).
(2) Instant noodles -- the kind in the cups.
(3) Tea -- green, orange pekoe, spearamint, crysanthemum...it's all good!