Like many, I've only seen the first three instalments of the OVA. Of these, the first episode is a sort of introduction to the series and is almost identical to the first episode of the TV series (though for fans it's different enough to watch anyway). However the second and third episodes are completely separate stories which were not repeated in any of the series (I have been told the 4th and 5th episodes are also different stories). They work very well in filling out the back story of the series, especially the main character's relationship with his 'sisters'.
The animation is fine for it's time, not many cel repeats, movement is smooth and overall well drawn. Happy lesson definitely has it's own 'look' which continues through both sets of OVAs and both series. Surprisingly little use of 'chibis' for a comedy, but big on outlandish facial distortions with very large expression-filled eyes. It works very nicely.
The sound is a definite plus-point: the voice acting is well done - especially the main character Chitose (voiced by Kishio Daisuke), who for once in a harem comedy is not given a whiny 'emo' voice. Also of note are the uber-energized talents of Sasajima Kaoru and Koyama Kimiko playing the voices of the P.E. teacher and Art teacher - genki to the max! The music is very good (the happy lesson soundtracks -[all of them!] were the first anime tracks I actually got hold of separately to listen to) - if you like, well, happy music I suppose :)
The story is a harem-type story with a definite twist - as the main character's been forced to share his own house with five of his teachers, leaving him sleeping on the couch. Happy lesson is all about comedy, and yes it uses the cliched stories we all know on occasion (for instance the obligatory 'beach' and 'open-air bath' stories here), but it does manage to add it's own unique flavour to even the most hackneyed of storylines, and it does it with a certain amount of style.
The characters are all well-defined stereotypes, but with so many main characters I think that's a good move, as you get to know who's who very quickly. This OVA series doesn't have time for much development of the characters (apart from a noticeable change of outlook for the main character), but don't forget there are two full series and another OVA following this! The characters are held to type very well, with no unexpected and illogical personality switches (often the case in more sloppily-made series)
This OVA and the following series, although appearing at first glance to be a stereotypical harem comedy, has gathered quite a cult following, and for good reason: what it sets out to do, it does extremely well. It's funny, entertaining, well made and well thought-out. I like to see new and groundbreaking material whenever possible and get tired of the same-old stuff, but somehow this series still managed to suck me in completely and leave me with a silly smile on my face at the end. I can't explain it better than to say that it's just one of those things that's more than the sum of it's parts.