Kwikspell courses are (probably) useful to some wizards, but not to squibs.
The Kwikspell course literature comes with two testimonials.
Madam Z. Nettles of Topsham writes:
‘I had no memory for incantations and my potions were a family joke! Now, after a Kwikspell course, I am the centre of attention at parties and friends beg for the recipe of my Scintillation Solution!’
Warlock D. J. Prod of Didsbury says:
‘My wife used to sneer at my feeble charms but one month into your fabulous Kwikspell course I succeeded in turning her into a yak! Thank you, Kwikspell!’
Harry Potter and the Chamber of Secrets
and states that they're useful for witches and wizards whose magical abilities are poor.
Kwikspell is an all-new, fail-safe, quick-result, easy-learn course.
Hundreds of witches and wizards have benefited from the Kwikspell
method!”
Harry Potter and the Chamber of Secrets
Interestingly, D. J. Prod appears to have gone from zero to hero with Kwikspell, having gone from 'feeble at charms' to warlock. What we don't know is whether these testimonials are accurate, but it's notable that both quotes come from wizards with poor skills rather than (former) squibs.
Like most correspondence courses, they're probably most effective if you apply yourself and have some natural ability. JKR confirmed that Filch's course was utterly ineffective, but that doesn't mean that it doesn't work for everyone, just that it didn't work for him (presumably because no courses will help a squib to become magical).
Filch has carved himself a niche at Hogwarts and Arabella Figg
operates as Dumbledore’s liaison between the magical and Muggle
worlds. Neither of these characters can perform magic (Filch’s
Kwikspell course never worked), but they still function within the
wizarding world because they have access to certain magical objects
and creatures that can help them
J.K.Rowling Official Site -Extra Stuff SQUIBS
It's also worth mentioning that another of Kwikspell's courses ("Wallop the Wombat Revision Course") is recommended by the Ministry of Magic itself to those who fail to achieve a passing grade in their W.O.M.B.A.T. tests. It seems highly unlikely that the Ministry would suggest people take courses from a company that offers courses that are known to be worthless scams.
(Do not be disheartened. You have only narrowly failed this most
demanding examination, and with a little further study would be sure
to pass. You are to be congratulated on a good attempt. We recommend a
crammer course of evening classes. Send an owl for full details of the
Wallop the Wombat Revision Course, available from the Kwikspell
Company.
HP: Lexicon - WOMBAT Tests
Additionally, we have evidence (from JKR herself) that other "correspondence courses" are effective in teaching inexperienced wizards and witches.
Occasionally, too, the magical community in a given country is tiny or far-flung and correspondence courses have been found a more cost-effective means of educating the young.
Wizarding Schools - Pottermore
As to how the course actually functions, we learn in the Gameboy Colour game Harry Potter and the Chamber of Secrets that lesson #1 of the Kwikspell course ("Learn Magic Fast with Kwikspell!") covers
- The appropriate way to hold your wand
- The incantation for the Skurge (scouring) charm.
Which strongly suggests that the course comprises of one basic wizarding technique and one spell each lesson.