IB is one of the best if not the best. Charts are not very good but in all other respects it is hard to find a better broker in terms of low commissions, range of markets, stability of platform etc etc. There are plenty of good low cost (or even free) charting packages that work with IB. To describe IB's platform as "third rate" is ridiculous.
The API is fairly easy to use and reliable. IMHO some things in the API could be done better, but it will get the job done. There is also FIX support available for higher volume traders. There is support for many different order types including OCA orders. It will probably meet your needs, but it's hard to be sure without knowing more about what you want to do.
IB is DMA so any slippage is what happens at the exchange - IB has nothing to do with slippage. Executions are fast. If you throw market orders at the HSI you will get slippage (sometimes a fair bit) regardless of which broker you use. HSI is a volatile and thin market.
IB's TWS runs on Windows, Linux and MAC.