We use the F005 and F006 here pretty much everywhere we need a simple micro for housekeeping and serial com and whatnot. The DACs, ADCs and comparators are fine and work as expected. We use the F120 for its EMIF interface; it's convenient to put FPGA resources in the micro's XDATA space. I built a couple of home projects using the F314 (no analog) and it's a pretty great little micro too. The F320 and F340 parts with USB work as advertised and if you want to do real Full-Speed USB (not just to make your device look like a serial port over USB) then there's nothing easier.

For the older families, note that the built-in oscillator frequency tolerance is quite wide so you will need to use an external crystal or oscillator if you want your UART to work. Newer families have much tighter oscillator tolerance so the external part is not necessary.

The JTAG and C2 interfaces (depends on which family) both use the same very inexpensive USB programming/debug dongle and they are easy to wire up in your circuit. As Eric says, just follow the eval board schematic and it just works. The F005 and F006 use JTAG and we regularly put them in the same chain as Xilinx FPGAs without issues.

SiLabs stuff is pretty great.

I just hope nobody buys them out and wrecks the company.

-a