Take most of the following with a grain of salt. I don't trade it yet, but I have done some preliminary investigations. I am not yet ready to go, so I haven't followed up......
IMHO there is not much doubt that the Kospi is the best equity futures market outside of the US.
I think I saw a table in one of the futures magazines that it is the 4th most liquid equity futures market in the world.
Depth is great. As I write, there are 1200 by 600 contracts within 5 spreads of best. Contracts are small - worth about US$35k each, but the thing turns over between 150k and 200k contracts per day. It trades electronically.
The main problem with it is getting set up. My understanding is that you first need to apply for foreign investor status, then there are a few other steps you need to go thru before being able to trade.
There are no big international brokers operating there (apart from the institutional guys - Merrill etc - which don't want retail business), but the local brokers (Samsung, Daewoo, Hyundai, LG)are well regarded, and, I believe, well capitalised. 4 or 5 of them offer on-line trading, in English, but all are browser based. Fees tend to be 1-2 bps per side.
Intraday charts look very similar to ES where either intraday trends tend to persist, or the market tends to trade well at support levels. I would suggest if you can successfully trade emini, you'll have similar or better success with Kospi.
I have seen a discussion on the IB website about the possibility of IB going into Korea, but it doesn't look likely short term. def will have a better idea.
JM2c