I have managed to learn some Estonian from this course, but there are some really serious flaws which make the course extremely difficult, confusing and frustrating to use.
The major difficulty is with the descriptions of the grammar, which are often either unclear, misleading or even quite wrong. For example we are told that the past tense is formed from the -da infinitive. In fact it is formed from the -ma infinitive. Until I had realised this I was very confused by an apparently huge list of exceptions which result from trying to build the past tense from the -da infinitive! In other instances a grammatical point is defined very vaguely where the simple statement of a firm rule would have been much more useful - for example the description of the genitive plural, Unit 9.
The vocabulary lists for each unit sometimes omit words used in the unit and some lists even have words which are not used at all! The vocabulary lists for Unit 7 are particularly sloppily constructed, with some words not even included in the usually reliable glossary at the back of the book.
Some dialogues have idioms or grammatical constructions which are totally unexplained or are only introduced later in the book. This leaves the learner confused and undermines confidence.
The answers to the exercises are full of dreadful mistakes: sometimes they miss the point of the exercise, sometimes they are wrong, and - worst of all - sometimes they include vocabulary, grammar or constructions not yet introduced. In the latter case students are confronted by a set of model solutions which they could not possibly have come up with and which in some cases are not even comprehensible. This too undermines confidence and is certainly the most serious flaw of the course. The exercises do not consolidate what has been learnt, which is what they ought to be doing.
All in all this is a poor effort, sloppy, inaccurate and ill-conceived.