I use a Roland GR700 and couldn't do without it as far as recording goes.
I use it for everything, like drums, bass, keys, synth etc... Plus you can use it to trigger your recording software or trigger samples by hitting certain key combo's.
When you use it live you can fade in the synth in the background so you can play chords and have soft strings in the background or whatever you want.
As far as the sounds go, it's midi, so it doesn't matter what it has onboard.
You can plug it into any MIDI keyboard, sound module or VSTi synth.