Amps, not so much mostly because I haven't bought a new amp since the 90's and most used ones don't come with one. Everything else, depends. Vacuum cleaner? Ya gotta be kidding. VCR? When all else fails....
On day 1, I always try to figure out the features on my own. Months later, I'll browse the manual if I have a question or just to see what some of the other features are. I almost always find it does something I never knew! It's like getting that new thing all over again.
I love reading manuals and have read many from products that I do not and will never own. It is fun. This morning I even read the manual to an electric shaver that @Killed_by_Death posted in another thread. That thing has more cutting teeth than I thought it would.
Currently I only have the manual to one item of my gear in print, but I will have links saved to where the manuals are online. I guess I'm a little strange in that, I'll save bookmarks to where the specs are for an instrument or amp, as well as the manuals. Didn't help me much with my SR1800E, because its page disappeared after it was discontinued.
If it doesn't come with one, I'll download & print one off. You never know what fussy little thing you need to be aware of, & occasionally I'll sell something to a relative noob. Being able to hand them a manual can be quite helpful to them.
Whenever possible, I download the PDF manual to everything I own. I have a folder on my comp labeled "Manuals" with different categories and items inside. Everything from my amp, to my cars, to my electric razor, to my TV remote, to my floor fan, etc. Great reference for learning things you might not know about an item, as RyanOh said. "Hey, I didn't know it could do that!" Also for parts, contact info, in case of breakage. Special notes or instructions to myself I may have learned on my own go in these, too. If you start using PDF's, especially on old items you've had for awhile, go online first and see if there are any updates to the manual.
Yeah, Handbook for the Recently Deceased from Beetlejuice. Had the hardcopy. Darn thing read like stereo instructions. But I found the passage that Otho used in the movie to raise the Maitlands from the dead. And I thought, hey, I'll try this on my mom! "Hands vermillion, start of five. Bright cotillion, raven's dive. Nightshade's promise, spirit's strive. To the living now let the dead come alive." I got nothing. Then suddenly I remembered, oops (headsmack), she's still alive! Oh, silly me. A minute later I got a phone call from her saying, "What do you want?" And with that, I'm having one final beer and going to bed.
when i went through a midi phase in the 80's, my gear was complicated enough to make a manual necessary, and usually required additional documentation to get the most out of the gear... for my old tube amps, all i needed was a schematic, which was usually stapled inside the cabinet... i don't think i've ever read a manual for a guitar or bass.
I'm a technical writer by day; I quite literally write user documentation for a living. I not only read my manuals, but I save each one. I actually have a large tote box with every manual I've accumulated since college. Call it a point of professional pride. While some of them are...erm...not great, I can't fathom the shame I would feel if I ignored the user manuals. ~Nick von Nick P.S. Check out Moog's manuals. They are amazing!
I do the same as well...IME you can learn a lot more from the manual than just to operate a piece of equipment. A manual can offer some useful insights into the capabilities of the equipment relative to your requirements and the quality of the manual often reflects the quality of the design itself.
Not like I need a manual for most gear, although I will read them if they're new to me or have some feature I'm not sure about.