I have bought and sold many guitars. Any mod from the original detracts from its value IMO. Not only that, but if you do the mods, even if they vastly improve the instrument, you'll likely never recoup the money spent. I've seen this on everything from cheap $300 guitars to $7000 Les Pauls.
For me, it depends. I have a Strat that I gutted because the US standard parts sucked. I replaced everything with top-notch components, swapped out the plastic, and now it just flat-out rocks. I've easily got $2000 into it, and I'd be lucky if I could sell it for $800. I think that's dumb, because I haven't changed the wood, and Strats were designed to be modular like that!
Carve the wood, though, and I'm out. The only time I've done anything like that was with a second BlueBird where I needed to route the corners of the pickup routes in order to fit vintage HB1s.
In short, electronics don't bother me at all, but any alterations to the wood and I'm out.