Just completed the Polish tree.
I just completed the Polish tree in less than two weeks, with the majority being done the past few days. I am a a native Polish speaker but while living in North America right I went through the whole tree to help with spelling of some of the longer and less used words used in daily life. The tree was easy for and the first half of the tree was fairly easy for a native speaker. However in the second half, especially towards the end of the tree, the translations do need to be more broad in the acceptance of answers. This will probably be fixed in time when more people start to get towards the end. I am sure both the Ukrainian and Russian trees are the same way right now.
I will most likely continue with Russian and perhaps include Ukrainian later on.
Perhaps you could contribute by slowly redoing the exercises and suggesting as many more possible correct answers as you can. You can use the form or comments - to help others. Also, I regularly browse the "sentences" part of the forum and I send the newly suggested answers to the course creators, in case they are not sent by the form below each phrase. And the tree is being constantly filled with new acceptable answers, but it goes very slowly so may not be easily noticed.
The Polish tree is lengthy, which has more words and more exercises, so it should give you the basic understanding of the language with a lot of vocab. Without the immersion though you wont be able to expand the usage and various shortcuts in the language. Overall, I would rate this course as good.
I second that, it's good, actually surprisingly so and fairly mature as well. In fact, I'd already rate it better than the Italian, although I just started that one. Immersion appears to only make sense in case one really wants to dive into it, and on Duolingo for that matter. Personally I'm not missing it, but I know what it is and would certainly like to see it being added to PL one day, for those who want it. (This word list/vocab feature would make at least as good an addition, though. I'm really missing that one.)
As you're a native speaker of Polish -- and you just gave me a shock up there until I reached the second sentence ;-) -- maybe I could attach a rundown from a 'not so' native perspective, simply as regards expenditure of time: I'm not through the tree yet, but almost, with ~10 skills remaining. This'll take some time still due to parallel strengthening and some personal constraints, I'm however looking forward to be at level 15 when I finish. With my streak being (so far..) "flawless", i.e. I started Duolingo 61 days ago and did Polish from the get-go, you can easily see how long it takes: ~2 months for the tree, or maybe 10 weeks in my case, probably sounds about reasonable. And I suppose I was (or am) neither particularly fast, nor particularly slow. Rather average.
It's definitely fun, but the Russian course will have its own merits, @Klimovskikh.
Just an overview between Russian/Ukrainian/Polish. I spoke to my Ukrainian friend in Polish, with him speaking in Ukrainian. Much of it is the same, although western and eastern Ukraine have different dialects, with the eastern being closer to Russian. Russian is pretty much completely different from Polish. It would be best for a non-Slav speaker to pick with one and learn it thoroughly.