The first and most important thing: Duolingo does not charge the user money (at all) and is basically ad free. They have an explanation for this, that makes more sense when you realize the guy behind it all is also responsible for Recaptcha.
You can use it on the web. You can use it on a portable electronic device that has apps, such as a phone or a tablet. There are a few pictures -- very few. It breaks language learning up into phrase and sentence oriented activities: translating to or from your own/the target language, repeating in the target language, multiple choice, dictation, etc. Sometimes you are asked to type in the language; sometimes you assemble the answers from little tiles. If you miss more than a certain number of times (generally 3) in a section, you have to start over at the beginning. If you don't miss any, you earn extra points/lingots. You can (I have not yet) buy stuff with lingots, including lessons in swearing in the language you are learning and similar fun things.
I've never taken any Spanish, however, I've been around Dora the Explorer in the form of the show, books, computer/electronic games, etc., and other less well-branded bilingual Spanish-English stuff. I also have two cousins-by-marriage, one who was born and raised in Mexico; the other whose parents came from there. So it's not like I've had no exposure to the language at all (do menus count?); I knew stuff like the you don't need to actually supply the "I" or other subject of a sentence rule and stuff like that. I also have some amount of awareness of French, German and Dutch as learned languages, so there's obviously a lot of cognates, and I have a basic sense of word order variation to watch out for, and the need to adjust endings on adjectives and so forth. The lack of explicit grammar lessons bothers me not at all. I could see where a lack of some or all of this background could make for one helluva confusing encounter with Duolingo otherwise.
I did not have any mystery microphone problems; if there's a problem with the speech recognition on this thing, it is that it might be uncritically accepting of terrible pronunciation on my part and I would never know.
I have definitely quite painlessly picked up a smattering of grammar and vocabulary very quickly. I'm having a little trouble with prepositions, which is really unsurprising. I have trouble with prepositions all the time.
I decided, in a manic moment, to try out the French and German courses. (This is really bad. When I do stuff like this, and then go to my Dutch lesson, I wind up completely unable to produce any speech because I get so confused. But it is fun!) I tested out of a tiny amount of each, which is unsurprising. (I couldn't remember the word for bird! Good news -- neither could R.) But going over the early stuff is a good form of review.
Today, as part of writing this review, I thought I would see what other people are doing with Duolingo. And I ran into something really, really bizarre that I'm just not sure what to do with. Why are people running this thing with google translate up in another window?
There are a million (hyperbole) more detailed, thoughtful reviews of Duolingo out there. Consider this a vote of confidence that their approach has been well-deployed across at least the few languages I have some familiarity with, it is not frustrating, and it is excellent, game entertainment. So, you know, while you're waiting for your Frozen Free Fall lives to regenerate, you could do this.