I have also received a similar offer from Aclipse. From talking with them on the phone it seems the represent a few ESL opportunities in Korea. CDI (chungdahm institute), EPIK (English Program in Korea), and SMOE (Seoul Metropolitan Office of Education), the latter two being government funded. As far as I can tell it's legit.

I have found a lot of horror stories about CDI, but none that involve non-payment, mostly about badly run branches. So all I can recommend is decide which branch (area) you want to work at before applying and do your research. Overall it looks like CDI is one of the better private companies to work for in Korea (I've heard the best things about YBM, but again it depends what branch you go to). It looks like CDI works you the hardest but you also have the best chance for financial gain, so it really depends on your priorities.

EPIK was formally Koretta, Korean's version of the Japanese JET Program, however, like most Korean versions of Japanese things, a little less planned out. I haven't been able to find too much information as to the current situation of the institution but pre-2004 it looks pretty bad. They could have changed for the better, could not. However EPIK would place you in public schools so if that's what you're looking for this would probably be your best bet.

SMOE is for Seoul public institutions, I've heard bad things about the living situation but that's about it. I don't know too much about the working side of SMOE but again it's government funded so they have a little less leeway to do shady things, although the possibility is obviously still there.

I haven't heard anything about Bridge to Korea but their website looks a little sketchy to me. I can just recommend to do your homework and if you choose one of the national ones know where you want to go and look into that branch before hand.