The other reviewers have alrady said pretty much everything. But just in case anyone is still wondering whether or not to purchase this product, let me outline some of the things I've found while playing:
1. Some of the words are translated wrongly. In one case, 'Er weiß' was translated as 'We know'. It actually means 'He knows'.
2. You are asked to listen to words and then spell them (for instance, in the hopscotch game for learning adjectives). However the sound quality on some of the clips is so poor that this is nearly impossible. 'Quietshend', meaning creaky, sounds more like 'Kriegshund', a breed of German Shepherd.
3. In the examinations in the end of each level, you are asked to complete sentences - for example, 'Der Himmel ist...?'. Except at no point in the game so far have you been told what the word 'Himmel' means.
4. You must complete 5 words in each of the minigames within a time limit. You cannot learn any new words until you have done this. This means you might end up spelling the words 'Toaster', 'Fernsehen', 'Computer', 'Telefon' and 'Mikrowelle' 25 times each, but not be allowed to learn any new nouns because you can't do all 5 in the same timeframe.
5. When you learn nouns, you do not learn which gender they are.
6. When learning conjunctions, they do not tell you the grammar that goes with them - such as which conjunctions send the verb to the end of the clause (it's 'aber ich bin einsam', but 'obwohl ich einsam bin').
7. When learning verbs, you only learn one or two forms of them. You learn 'Er isst' ('he eats') and 'gegessen' ('eaten'), but not 'Ich esse' ('I eat'), or 'essen' ('to eat').
8. The order in which you learn words seems bizarre. 'Beetle' and 'underground train' are learnt in level 1, but the numbers 1-10 aren't needed until level 4.
9. In the examinations, you are presented with a sentence and told to fill in the blanks. A voice reads the sentence out to help you. Except they actually read out a completely different sentence. The words on the screen might say 'Ich mag Saft und Kaffee trinken', but the voice says 'Ich fahr mit meinem Fahrrad'...
10. Even in the parts written in English, there are grammatical errors! I've seen 'your' instead of 'you're' a few times...
11. The minigames are repetitive, and their only purpose seems to be teaching you how to spell words. You walk around a maze for 90 seconds finding the letters to spell out 'gegangen'. But it doesn't explain to you that gegangen is the perfect tense of gehen, 'to go', and that it takes sein instead of haben ('Ich bin gegangen', but 'Ich habe gafahren'). So you're left with a word that you can spell, but don't understand.
The end result is that the game is not much fun, and leaves you with almost no working knowledge of German. I would be very surprised if anyone learning German for the first time with this game could construct 5 sentences correctly, even after finishing level 5.
Should you buy this game? Sure. As long as you're only intending to end up with a list of German words you know how to spell, but not what they mean or how to use them.