I asked a co-worker what she thought about tomato bank and she said that you have to be careful and find out how secure the bank is. Tomato Savings Bank went bankrupt recently:
http://forums.eslcafe.com/korea/viewtopic.php?p=2692498 So if you are looking into Korean Savings Banks, you should see what they are rated for security. My co-worker said there were three levels and that Tomato Bank is the worst level. So, you may get a better interest rate, but your money may not be secure.
In the U.S., FDIC insured banks should reimburse you for up to $250,000 but in Korea, their version of this will insure you up to 50,000,000 won. You had better make sure your savings bank has this before giving them your money.
Anyway, I am trying to find out more on the internet, but have just got this information from a Korean co-worker who is responsible about her money. She has a lot of savings in 새마을, which looks like a kind of credit union, that is rated well for being secure.
http://www.kfcc.co.kr/ If anybody else has anything to contribute from reputable sources, that would be great!