Booleans are a bit of a funny beast.
I find they're best used on quite simple geometry that you can't really be bothered doing manually. Whenever you've got geometry that's going to take the longest, and that you REALLY can't be bothered with is the time they're usually least suitable. Mmm I know, just when you need em, you can't have em. When you can have em, you don't really need em often...

On a side note, with the guitar, if you keep using booleans, you may be better off just modelling half of it and then mirroring that to get the other half. This will especially help when it comes time to boolean the second part of any pair, since this is much more likely to fail than the first - perfect example was the cut-outs in the body.

With experiance, you get to know how much detail is needed when putting something together and how much can be added in after the fact. I say this because the notes look reasonably 'heavy' in geometry as compared to the rest of it.
But no matter, looking forward to seeing more of this 'tar.

Doing a search for booleans around the forum will give you a bit of an idea about the amount of pain they cause, and some of their weaknesses. Once apon a time I though you could build anything with 'em :snicker:....
Hope it helps,

Simon.