The Saleae analysers are both
They provide a very useful help to debug digital signal. For analog signal, I'm using a Rigol DS1102E Oscilloscope.
There's no malfunctioning circuit that I can't debug and fix with this great tool. Whatever I²C, SPI and UART, nothing resists it!
The initial Saleae logic analysers came with 8 or 16 channels. The new generation now features analog and logic trace for 4, 8 or 16 channels.
Read my review of the 1st generation logic analyser and of the 2nd generation analog and logic analyser.