Having spent at least some measurable time and effort on study in more than 10 languages, I've tried several methods and used several language learning products and systems. As you can probably imagine I've collected a rather extensive library of books and other learning materials.
I'll be the first to tell you that the majority of the language-learning products available on the market are utter rubbish. In fact, not only are they not good products to learn from... in many cases, I feel that they are actually harmful! Not only is memorizing vocabulary out-of-context a mostly useless activity... memorizing vocabulary lists and translations is actually actively working toward your own failure!
Therefore, when I find a good resource, I take note. If I try a product and hate it, I usually throw it in the garbage. (Giving it away would only introduce that problem to someone new.) But when I find a product that I like, I remember it and try it again in the future. And that is the case with Colloquial Lithuanian.
I've long had my eye on the Colloquial series of language learning books, but they're not available at any local bookstores in my area, and Amazon doesn't have "Look Inside" previews for them, so in spite of my curiosity I never tried one out. But with the dearth of good materials for Lithuanian, I took a chance on it, and I'm glad I did.
Why I like it
These books are clearly not aimed at beginners. But they do contain all the information you need. The difference is that where beginners books spend a great deal of time with hand-holding and coaching you at a snail's pace through simple vocabulary, the Colloquial book dedicates one chapter to very basic information and then jumps straight into dialogues.
Right out of the gate, I'm reading (and listening to) Lithuanian vocabulary as it would be actually used in a real-world conversation. That means that instead of trying to learn that sveikas means healthy, but it can also be a greeting like hello and then getting confused, I just read (and listen to) the dialog, and I see and hear the word and I understand that it's a greeting. Simple.
Moreover, because I'm reading and hearing, and learning conversation, I'm hearing responses adjacent to questions, greetings with greetings, goodbyes next to other goodbyes, etc. I will say sveikas in response to labai gerai. I know I will, because I've learned it that way. I won't be standing like an idiot trying to think of a response from my lessons.
This is also a very good way to learn any language that uses noun declension. For instance, the word kas means what, but I know that in most instances I will say ką (the accusative form) because I've seen the word ką about 10 times more often than kas. It's two declensions of the same word, but one is clearly more useful — and therefore more important to remember — than the other.
Colloquial Lithuanian is written by a pair of authors, one a native English speaker and the other a native Lithuanian speaker. This is a detail I'm particularly fond of, because both the English instruction and the Lithuanian portions are natural and correct, and the explanations are very clear. I don't know if this is true with every language for which there are Colloquial books. I hope so. I'll definitely try this series again for another language.