OK, first a couple of disclaimers. I'm using Babbel to study German, not Polish. So the experience may be slightly different for Polish, but I'm guessing it is about the same or pretty close. I figured I have plenty of resources for Polish (but would have checked out Polish here if they threw it in along with another language), enough stuff for Dutch, my own resources for Romanian (not much on the Web as it is a fairly minor language), am working with Anki for French, Italian and German, so I figured now is a good time to expand my knowledge of German. The second disclaimer is that I have only gone through a few lessons and spot-checked some others.
The site is not bad, but it moves along really slowly. It's probably good for somebody who is just starting out learning a language, but for voracious multi-lingual information processing people, it's going to move at a glacial pace. What we want is massive amounts of information coming in rapidly and then to know that some of it will stick to the wall and some of it will splatter off or slide downward and we can go back to it. Vocabulary comes in fairly slowly (just a few words a lesson), but to its credit, it is presented with simple sentences in context that are fairly easily decoded. There is not much discussion of grammar (there are a few grammar exercises, but you really don't get your hands dirty) and not a lot of oral usage other than repetition of the sentences.
There are a lot of matching exercises. I'm not a huge fan of matching exercises, but I guess they are OK as an introduction. Also, there are exercises where you have to type the names of what you are translating, and it takes off points if you make a typo while you are typing instead of grading the whole thing after you have typed it. So you have to be very careful typing. It does put in diacriticals (accent marks or special marks) automatically and you don't have to type some special keyboard. The good: it saves you time. The bad: you might not get the best knowledge of where the diacriticals are and aren't supposed to be. On the whole, though, I prefer them being put in without having to type them specially.
As you complete exercises, you earn "Babbel Points." But I can't find any explanation anywhere of what this means or what the points signify or do. It would be helpful if this was explained in the FAQ section.
There doesn't seem to be a whole lot of advanced material, though there is some. I would guess this is because most language learners are beginners. Probably some of the best stuff is in the course group called "Specials". You can review at an A2 level, and there are also some exercises on idioms and false friends. There are some more advanced narratives in the "Business" part of the language.
The first lesson in any given language is free so you can try it out, and then you have to pay to access the rest. I got in on a half-off deal, or rather, six months for the price of three. Six months is a good length of time to check something out so I'll probably report back again at the end of that period. You have to pay extra if you want another language. Of course, that is reasonable. But I'd like to see one of these multi-lingual pay sites offer a little of all their languages for a short period so you can try it out. Hardly ever do any of them do that, though.
One thing that turned me off is that the first lesson on "Animals" is "Animal Sounds." Who is ever going to need to communicate how to anthopomorphize a dog's bark or a cow's moo in a different language? That is pretty low on the priority list and should be the last lesson if at all. That is one thing that I learned from buying children's books in Poland. At first, I thought that children's books would help me learn like a child does. But then I realized that they talk about things that are really not high on a traveler's priority list whereas language courses think more about that. I really need to know how to ask where the bathroom is when I'm doing a pee dance. Or I need to know where to get food when I'm hungry, money when I need to buy something, a hostel when I need to crash, etc. Priorities need to be considered.
Another drawback is you have to email them to stop your subscription. I really don't like automatic renewals on anything. You have to give them a credit card number and then they charge you automatically. That's OK if you are certain you will keep on using it for a while (well, not really, because at some point you might not). But to add the added step of not being able to unsubscribe on the website and see that you are unsubscribed, and having to contact someone who works there so you can beg off from further payment, that's another layer of jerkdom.
Also, they sent me a survey recently and there were only two choices on everything, either I really love it or it is crappy (whatever feature they were asking me about). I rated almost everything poorly because they didn't give me more choices, but I might have gone a little higher if there had been more than two choices.
One peculiarity is that when viewing the site in Google Chrome (I prefer Chrome for language learning because of its translation features), it tells you that the site is in Spanish, and asks if you want to translate it from Spanish. There was no Spanish at all on any of the pages, just German and English. Maybe some of the HTML or programming comments are in Spanish, who knows. If you hit the button to "translate", nothing happens, but the dialog goes away.
I'm not going to tell you not to use this, but in my opinion, there is better stuff out there. If you get a deal on it (I paid a little over $25 for six months), it is probably not bad for the value. My general rule of thumb is that anything that gives you a deal for under ten bucks a month is worth trying out for a month or two if it looks like it could work. Six months is probably a longer tryout than I would usually go for, but the price was good. It won't hurt me to have a half a year of exposure to German, though I don't think I'll do it every day to wring my usage out of it like I have with other trials in the past.