You know the more shit I hear people say about the dispatch companies, the more I think "this is the job for me". They go on about how it's basically a marketing pitch for their textbooks and bundles of lessons, and that it's run like a host club. I understand the hours are murderous, but I've heard success stories of guys who have been popular with students, been good at teaching and have been able to make decent money.
OP, I don't know who you've talked, or if perhaps things have changed in the many years since I moved back stateside, but I do have very pertinent experience with what was called "dispatch" in those years and if it's anything like what I personally experienced, you aren't getting very accurate information.
The Dispatch Teacher racket was, simply put, a system that connected lazy government BOE departments with greedy private corporations to provide walking, talking gaijin teaching assistants in public schools. Nothing more, nothing less. In my case, the Osaka Prefectural BOE had two modes for hiring foreigners to work in schools, and primarily their high schools.
One was the "Direct Hire" gig, which was the holy grail for most useless expats slumming it through life. As the name would suggest, the Prefectural BOE would hire foreigners to work as NETs (Native English Teachers) in, generally, the higher-ranking, academically-advanced high schools. You were generally required to have at least a Master's degree and some sort of teaching qualifications/certs from back home. Depending on the school and head teacher you worked under, your duties might range from team-teaching to actually running the entire 40-80 student classroom on your own. You were an actual employee of the prefectural government, at that time were paid about 310,000 JPY/month (which was straight baller, since eikaiwa grunts were getting paid about 250k at the time), were entitled to all the paid holidays that Japanese teachers received and were enrolled in the national healthcare system. In Osaka, the NET department consisted of less than two dozen teachers for the entire prefecture, so as you can imagine, the job was very, very difficult to get, extremely competitive and involved a very lengthy application and interview process. Unless you had some ridiculous degrees, credentials and experience, you needed an inside connection at the BOE, and usually someone very high up.
The other was the Dispatch Teacher gig. Since most large BOEs didn't want to personally deal with the hundreds of gaijin teachers needed to fill all the prefectural classrooms, they would contract with private companies to provide warm bodies. Most of the large eikaiwa outfits at the time (like AEON) developed departments for just this purpose. ECC, for example, developed the "Corporate Sales" division. Basically, the BOE would pay AEON/ECC the full 310,000/month to provide them a warm body five days a week, AEON/ECC would put out ads for "ALT" (Assistant Language Teacher) gigs, and then AEON/ECC would pay those desperate fools about 230,000/month max on a per diem basis, "dispatch" you to a public school (and usually not a very good one) and pocket the rest of the money. You were not enrolled in the NHS. You received zero paid holidays (again, company pocketed that money). If there was no school because of a national holiday, you didn't get paid for that day (so you usually didn't get the full 230k).
Also, since it was basically understood that these ALT "teachers" were going to be practically useless, they were generally used as tape-players with a pulse. Often times, dispatch ALTs would work alongside JET ALTs in some of the prefecture's shittiest schools with the shittiest students, doing the ever-important tasks of singing the alphabet or role playing dialogue from a textbook that even 5yo SPED kids would find retarded. And as if that wasn't demeaning enough, the dispatch ALT would soon enough figure out that the JET ALT, unlike him/her, was an actual government employee and thus entitled to all the pay and benefits that his/her eikaiwa dispatch company was robbing him/her of. So in other words, the 22yo JET jagoff, for whom this was his/her first job right out of unit, was getting paid the full 310k, had real health insurance, got all the paid holidays and probably got a semi-subsidized apartment, too. But because both of these ALTs were almost always utterly useless, in the case of the prefecture's more advanced/prestigious high schools, the head teachers would request the BOE send them an actual NET to teach the actual classes, or at least un-fuck whatever the two ALTs had fucked up, and the dispatch/JET idiots were relegated to the back of the classroom to do coloring books or cut out paper snowflakes or some shit like that.
So again, if you're hoping to do actual teaching, dispatch is not the way to do it. If you're financially set and none of this bothers you and you're just looking for a technicality to pad your resume back home where they won't know what "dispatch ALT" means, then by all means have a great time. Just know what you're getting into.