Oh dear, this is what I do (I think as I get them prepared by my accountant).
1. No deductions from salary for my contributions and my employer contributions are taxable as wages.
2. You need to fill out form 8621 which relates to a passive foreign investment company. That means that you have to mark to market your funds returns every year and any gain is taxable as income.
3. As the IRS does not recognize the MPF as a pension scheme (lack of tax treaty for a start), I can't imagine why withdrawing the money when you leave should cause problems.
4. If you work for a US company try and find if you can set up an alternative scheme to your MPF, eg a 401k. Otherwise set up an IRA, US$5000 per annum versus c. US$3,050 which is the maximum compulsory amount you can put into your MPF. Charles Schwab offer this service for people living in HK, E-trade don't. In order to do this you must have taxable income (i.e, be above the c. US$90k threshold).
Generally avoid any form of non-US collective mutual fund, especially if it wrapped up in an insurance policy unless you have it cleared with a professional US tax advisor. The reason is that in the US mutual funds distribute income and tax forms to their holders every year. Non-us ones don't and allow in most cases for people to defer the tax until they exit their investment.