Having read all 5 Tanya Grotter books (there are already 5 at time of this writing) I can confirm that it's definitely a parody and not a simple rip-off. Moreover, it's quite enjoyable reading. The language is rich and colorful, the content is heavily based on Russian folklore and culture, including, for example, such characters as Baba Yaga, Zhar-ptitza (Firebird) and even Porutchik Rzhevsky.

All the similarities with Harry Potter books are purely intentional and often are represented as "mirroring", such as swapping genders and physical features (the hero and two main adversaries become female; the annoying know-it-all student becomes male; Tanya's aunt is fat, and uncle is thin; wands are replaced by rings - which, in a way, is also gender-switching !).

Other aspects are exaggerated to the level of caricature, resulting in a zany and weird atmosphere. The magical school Tibidokhs is also a behaviour-correction facility for criminally-minded young wizards - and a deadly dangerous place, full of nasty uncontrollable creatures, even some of the employees wouldn't think twice before cursing a student with a fatal irreversible curse (I want to work there !) Instead of Quidditch there is wacky Draconbol (Dragonball) which involves the players flying on just about anything (most often - various musical instruments) and throwing special balls into the mouth of a dragon, often ending up swallowed by the dragon. There is even a tongue in cheek running joke featuring Harry Potter himself !

The similarities end with the scenery, characters and the basic premise - as for the plot, it's completely original, quite intriguing and well thought out.

The 4-houses structure didn't make it to the Russian books, having been replaced with the more traditional division into black magic and white magic departments. This is actually true to the spirit of Harry Potter books, since one of the houses (Slytherin) strongly represents the "dark side" (note that I personally disagree with stereotyping Slytherin as simply the place for everything evil)

But back to Tanya Grotter. The most notable feature of the books is humor. They are not meant to be philosophical , deep or dark, their goal is simply to entertain. I see that as a fault - the books don't convey a sense of development, the characters are not developing, every next book is just like the others - and after the feeling of novelty wears off, they seem more and more shallow, somewhat repetitive and less funny.

Overall "Tanya" is definitely worth reading - at least, the first 2-3 books; but it's far from being a replacement for Harry Potter. If anything, it can be perceived as free advertisement - surely Tanya fans would be curious about the real thing, and I can hardly imagine the readers to actually prefer Tanya to Harry. In my opinion, "Tanya Grotter" books constitute no more threat to Harry than fan fiction. Let a thousand flowers bloom !