I suppose there are as many opinions about sharpening as there are folks using knives! My perspective on sharpening is a maybe a bit different as I'm a chef; for years I hung out my shingle as a sharpener on the side and did some work for a high end Japanese knife vendor. But while my specialty (if you will) is Japanese kitchen knives the method is universal- no matter the blade you must grind the bevels til they meet [usually in the middle] and then remove the burr. Finer stones refine the edge but don't really make it 'sharper'. A knife should cleanly shave coming right off the arato (coarse) stone once you remove the burr.
Personally I don't advise oil stones. They're simply too slow compared to a water stone, and once they dish there's not much to do with them but toss 'em out. They can do a good job of course they're just not ideal IMO.
I'm not crazy about diamonds either except for setting a bevel. IMO an Atoma 140 is the perfect arato and the DMT XXC is a close second. Diamonds are great for some 'super steels' that are very hard and/or very abrasive resistant. But they'll wear out pretty fast on softer steels like 440C.
Belts work very well! But there's a learning curve; not a steep one but as others have noted you need to use a light touch to avoid removing too much steel. You can't add it back on! A cheap grinder like the Harbor Freight runs to fast and risks burning the edge and removing the temper.
For simple maintenance the Spyderco Sharpmaker is super. But if you let the edge get to dull you're in for a long day trying to reprofile it on the stock ceramic rods. It's best for touch-ups between regular sharpenings.
A mousepad and sandpaper is certainly viable but only for convex edges. I'm not a big fan of convex although that's just my personal opinion. We all bring biases along with us and mine is this- I like the same kind of edge on a hunting or bushcraft knife as I do on a kitchen knife. Convex edges kind of "swim" through the cut with food and I hate that. YMMV.
For an entry level device that's not super spendy, and if it's for knives under 5" or so, the Lansky is pretty good. I think the Gatco is a bit better and the KME better still. For a bit more money the Edge Pro Apex is hard to beat.
Free hand with water stones is very versatile. Once you master it you can do everything from an axe to Yanagi-ba. Getting good will take somewhere between a couple months and a couple decades, it depends on your aptitude and commitment.
Some of the upscale jig/guided systems have a wide selection of superb water stones available. The Edge Pro line and the Wicked Edge both can be had with Shapton Pro Stones, Shapton GlassStones, Naniwa Chocera/Pro stones, DMT plates, Atoma diamond plates, Sigma Power, Beston, etc. If you plan to do a lot of sharpening work it can be well worth the money.
Sharpening can transcend being something you do because you need a sharp knife; you can go "down the rabbit hole" and let it become a serious obsession! I have perhaps $2,500 worth of Japanese kitchen knives in my work case but at least $3,500 worth of water stones! A good Japanese natural (ie mined from a mountain) can cost as much as a compact car. You don't have to spend a ton of course but it can become a hobby in its own right.
