Korg SOS please help

[Alissymccracken]

Hi to the people who read this message, in thinking about buying a korg SOS early next week, but there's one thing I really want to know. This may seem like a dumb question, but I've never used one before. The question is: say you want to record a little guitar part half way through what you've already got, how can you do that? Is there a way to do it without waiting for the track to play through? I hope this makes sense?
Also is this device good for making reasonably decent sounding tracks? What kinds of things can you do with the software included?
Thanks to anyone and everyone who can help me with my questions. Sorry it's so badly phrased!
Peace!