HK has no capital gains tax. That means for most of the real world you wouldn't have to pay taxes on your gains. However, as I assume you are American - you are out of luck. (I'm in the same boat on taxes but since I am unable to trade HSI for my own account, I can't tell you how you would handle it).
HSI: I've been trading HK for about 8 years. Volume on the futures is about 15-25K per day. Although depth is usually thin on top of the books, however, getting filled is not a problem. The volatility of the index makes it interesting to trade. I'd recommend trading it but it really would depend on your style. ie. If your style is to scalp for a few ticks you probably will get burned when your stops can get blown out by gaps in the market. However, if you are good at recognizing trends and/or support resistance levels, you may do very well.
Speed & prices are very good. IB's price feed is direct from the exchange. Our clients get the same feed as our prop side. We charge 30 HKD all in which includes exchange and regulatory fees of I think 11.50 HKD. (To put the fee in perspective, when using outside brokers for option trades on our prop side we are charged 25 HKD per contract w/o clearing and exchange fees). Each tick is 50 HKD thus you'll need two ticks to make a profit. The standard deviation is a bit over 150 points.
I know a number of you are trading the index from the states and would be interested in your thoughts/comments as well.
The best thing to do would be to watch the market for a while and make your own determination. Right now for trading accounts, the HKFE data feed is free. (I hear rumblings from the exchange that they may start charging for data so this may change. my issues with exchange management is a topic for another day).