I'm already level 6 in Italian on Duolingo. I can speak some Italian. If I complete the lessons and internalize all of it, can I be sure that I would be understood in Italy? Will I not sound like a "book"?
TL;DR: With only Duolingo you will sound not like a book, but like a tourist who has somehow memorized a very long phrasebook. However Duolingo together with other learning aids can be a very powerful tool.
I use Duolingo for other languages (German and Russian) and I think it is an invaluable and powerful resource. However it is not sufficient by itself to learn a language. In particular there are three things that Duolingo lacks
Grammar. Duolingo goes from downright awful to barely sufficient for learning the grammar. In this it shows its roots as a tool for learning English (a language in which most of the parts of grammar that cannot be figured out on one's own are advanced topics). A good grammar book with exercises is essential here to complement. Seriously, this is the part where Duolingo sucks.
Conversation. As all self-learning methods, Duolingo cannot replicate the challenge of being actively engaged in conversation with someone. Unfortunately there's not much to do about it, except finding someone to practice with (possibly a native speaker).
Reading. Here Duolingo is serviceable, but you should actively seek articles and short stories compatible with your learning level. The 1-2 sentences of Duolingo exercises just don't cut the mustard.
On the other hand Duolingo is very useful for other parts of the learning process:
- Learning words. Practice practice practice. After doing Duolingo's course you will have at least a sufficient starting vocabulary.
- Example sentences Duolingo also makes you memorize sentences, which is very useful for cementing the grammar rules by knowing many examples. However you have to seek somewhere else to learn the actual rules.
In practice I tend to read the relevant chapters of the grammar books before doing Duolingo's skills and it seems to be working fine so far.