I really liked the Rosetta Stone system, but there are a couple of issues you should have in mind if you are going to use this product:
1) There are no translations of the words you are learning. Most of the time this is not an issue. For example, you'll see a picture of a dog with a Polish word written underneath ("pies"). But some of the time I look at the picture and the word underneath and I have a hard time figuring out what they're trying to teach me. Sometimes I made the wrong assumption. I went to Poland and had several misunderstandings because I thought a word meant one things, when it really meant another. For example, I looked at an image in Rosetta Stone with some people having a party at the beach, playing the guitar, dancing, and talking at night. Couldn't make out at all that the word I was supposed to learn was 'bonfire' (there was a small bonfire on the far lower right of the image). To avoid further misunderstandings, I started looking up every new word I saw in a Polish-English online dictionary (I recommend the University of Pittsburgh online Polish dictionary, though google translate was also handy). In addition, there are a ton of words in a language that are hard to represent with an image; especially abstract concepts. Once I had my dictionary handy, I didn't have more misunderstandings though.
2) The Polish language is rich with not only conjugation of verbs, but declinations of nouns (nouns also change based on their function in the sentence). The declination of nouns is extremely difficult to grasp in a flash-card system such as this. I was totally lost until I decided to study declinations properly. For example, the Polish word for 'window' can be 'okno', 'okna', 'oknu', 'oknem', 'oknie', 'okien', etc, based on the word's role in the sentence, whether it's singular or plural, or whether we're talking about 1, 2-4, or 5+ of them (gets complicated, doesn't it?). Looking at random phrases, it's impossible to figure out why nouns keep changing. Look at the word for 'car', which is 'Samochod': "She drives a car" is "Ona prowadzi samochod", but "She doesn't drive a car" is "Ona nie prowadzi samochodu". 1 bowl is "1 miska", 2 bowls is "2 miski", and 6 bowls is "6 misek". If you don't know the rules, it'll just look like the nouns are changing randomly throughout. Not knowing this, I reached an impasse with the lessons, so took about a month to learn about noun declinations and basic rules of gender and number before continuing my with Rosetta Stone. Once I these, the rest of the Rosetta Stone lessons made sense.
3) There's a concept in Polish that is still hard to grasp for me, but used fairly often: The use of the word "sie". I think it's a reflexive adverb, but where it's added in a sentence is tricky. I think you're supposed to put it after the object the verb is affecting, but not always. Knowing Spanish, I think it's similar to the word "te". It only appears in conjunction with certain verbs. The point is, Rosetta Stone doesn't shed any light into the intricacies of a language; such as this one. Not knowing why things are the way they are leads to parroting phrases. Makes it harder to construct sentences of your own.
However, let me say that those 3 issues are problems no matter what method you're using to learn Polish. Polish is one of the most challenging (messed up?) languages you can learn. There really isn't an easy way. I thought going to Poland and immersing myself would make it easier, but it really didn't. It's a very complicated language. I was told that no one, not even in Poland, really understands the language. Funny anecdote: While in Poland, I heard a complete sentence, of about 8 words, without a single vowel in it! I know Spanish, English, Mandarin, and a little Japanese and I can tell you Polish is the most challenging I've seen. Another example of the difficulty: the word "from" in English has 14 different Polish counterparts according to context (you have to know which one to use in which context).
In terms of having a tool to practice and progress through Polish, Rosetta Stone's strength is that it's fun and interesting. I look forward to running Rosetta Stone every single time. It makes me learn and practice Polish every day. At the end of the day, that's the most important thing, isn't it? To keep you trying. To keep you interested. I would've given up using books or audio tapes due to the difficulty of Polish (it also helped me that it is expensive and I want to get my money's worth). I rather do Rosetta Stone than read through books or just listen to audio tapes. For example, learning the phrase "that smells bad" and seeing a picture of a guy holding up a sock and having a nasty expression in his face was hilarious! The pictures do provide good context. Often I remember words and phrases, because I remember the image that came up with it.
Rosetta Stone comes with audio CDs. I use them while driving. They are invaluable in order to fully learn the vocabulary and phrases of the language, but more importantly, so your brain learns to make out Polish when you hear it. At the beginning all the words in a sentence seem to mesh together. You can't tell where one word ends and another begins, but the audio CDs really, really help with that. This is particularly important in Polish as there are words that are hardly audible (such as "w" and "z" - yes, those are words, and each has multiple meanings based on context).