So, is crafting p. much an all-or-nothing affair? Either you go for it and get a couple skills and some feats, or you avoid it? Or is it worth dipping into one or two crafting skills?
Nah, you can get away with just the skills if you want. Everything you make will be better than the stuff you find. Adding feats will just make the gear godlike.
I presume the usefulness of crafting skills vary wildly depending on the rest of your build?
Not really. Any kind of build can benefit from any of the crafting skills. There's no drawback from picking any of them, ever.
I thought some of them were strictly usable for certain items. Guess not. What is the general difference between them, then?
Well if you're going for something quite specific like a build that uses Lasers then you'll want Electronics to make your Laser guns, but that's as specific as it gets.
Here's a general breakdown for the skills, I guess:
Mechanics is for guns, melee weapons and armor
Biology is for health and psi hypos as well as drugs that buff you. Some animal parts may require biology for crafting armor out of said parts, like the psi beetle carapace for the Psion armor.
Chemistry is for explosive materials
Electronic is for shields, cloaking devices, psi headbands, Lasers, EMP mines and bombs and I want to say chemical pistols as well
Tailoring is for armor, shoes and balaclavas. It uses cloth and animal hides. Think of lighter armor materials as opposed to Mechanics which deals in heavier armor materials like metals.
There's something useful for every build, since every build can use good armor, healing items, energy shields etc.
The thing is that if you want to craft armor then you'll need both Mechanics and Tailoring. If you want to make badass melee weapons that have mods so that they also do electrical damage then you'll need Mechanics + Electronics. All the crafting skills play off of each other.