I have done Duolingo - i did the free version, as i am too stingy to pay for it.  I found that i got stuck at a particular stage when i was giving the correct answers, but Duolingo didn't accept it.  With the free version you have a certain number of lives and so i would lose my streak .... choice ... do i pay, or do i stop?  i stopped :(

I tried to for a little while leading up to my trip to Japan. It was fun but I didn't keep up with it and don't recall most of what I learned. I held onto things I aaa exposed to out there a lot better though. Suppose I just don't learn the way they teach as well.

I actually did not know about it until recently, but now that I do I think I may try to learn Vietnamese. My girlfriend is half Vietnamese (her mother is Vietnamese, her father American) but cannot speak a word of it. I think it would be hilarious if I learned it and could speak to her mother with my gf being oblivious to what we are talking about. :P

I learned 2 languages off of there but without practicing I forgot. 

re-reading this thread has inspired me to download duolingo again and see if I can give Spanish another go.

Although reading through the replies i'm also tempted to try some languages ive never even attempted before lol.

No

i cant even remember how many times ive tried to use this app lol but i always end up losing interest!

i think ive learned a bit from it though

I'm currently learning how to speak Irish Gealic on Duolingo, it's been a struggle to stay motivated, but I'm trying to force myself to continue learning, and one day I know I'll finally learn it lol.

It’s definitely very easy to use and I think easy to stay motivated to use too.
I just run out of time in the days but used to try and just get half hour an evening on there and definitely helped a lot.

I have been using it on and off for a few years to learn italian. My problem is staying consistent. I get a two week streak then life happens and I forgot about it and ignore all the reminders it sends me

Using an app like DuoLingo to learn a new language is cheap, easy and convenient. I'll be honest though I would prefer to learn a language in person as I feel that is how I learn best. I've tried the app a couple of times but not really made huge amounts of progress with it.

I’ve been learning French and Spanish for like 6 years. I can’t remember to keep up, but I really want to learn these two languages bad! But just time it’s not enough hours in the day. 

I use it on and off. I really need to stick to doing it daily. I agree with many of the comments and general consensus about immersion driving the learning. My wife's native language is Portuguese (which I practice in Duolingo) - definitely helps living with a native speaker.