BP compatibility: except newest features introduced in Delphy it is working nice. Few features require special compilation options, few are unimplemented on purpose. But all of them are the ones which your better not use anyway - gpc does it more clearly - it implements Extended Pascal standard. You should start reading info pages for gpc it is all described there.
FPC: there was a discussion on this list and as I remember the word was that it is not implementing so many extended pascal features and is not
It doesn't even support standard Pascal. FPC is targeted at Borland Pascal and Delphi.
The development version (which is quite stable atm) nearly implements the entire Delphi syntax. (including Kylix and D6 extensions)
very optimising.
FPC is not as optimizing as GCC in extreme cases, but is generally something like -O without tail recursion, which is fine except for specific applications.
I did not try that one - for me compatibility with standard library and ease of linking with c, c++ is important.
FPC can link with c and C++.
gpc is based on a gcc backend, so all its features are easily accessible (just write the wrapper or link directly!).
FPC can do exactly the same, EXCEPT that it always requires Pascal headers.
FPC or GPC depends on your task and system. In general: - Non x86? Then GPC. (m68k FPC in beta) - Extended Pascal? -> GPC - Delphi code ? -> FPC