There are some good answers already, but I don't see any that mention this kind of direct experience: I play violin; I started at the age of 6, and I had violin lessons more or less continuously from that point until I was about 17. I continued playing regularly for some years after that; I'm 28 now and still play occasionally but I have less time for it, sadly. At any rate, I know how long it took me to learn, and I have testimonials from my parents (and later from my flatmates) regarding the general quality of the sounds produced by someone learning to play the violin.
I can say, fairly categorically, that violin is not a beginner-friendly instrument.
For a time, you likely won't be playing recognisable tunes; it's much easier to focus on a particular skill with drills and exercises before moving on to add the next. This "training-wheels" approach will help you get to grips with all the required skills faster, but you won't be able to play actual tunes until you've got them all down. For example, when you're learning to control the bow - the right amount of pressure, the right speed, the right place on the string, the right angle, how far to move it, how to vary the speed and pressure during a stroke, how to switch strings cleanly - you'll find it easier to stick to exercises that let you ignore the fingering, which means you'll be using only four notes. That sounds boring, but until you get your bow control sorted out, any notes you do play will be scratchy, or faltering, or poorly-timed, or interspersed with random clashing as you accidentally catch the other strings, so you're really better off getting the hang of it before trying to carry a tune.
Another aspect of violin that makes it difficult is that it has a smooth fingerboard. A guitar has frets, which gives you an inch or so in which to place your finger and be sure of getting the right note. On a violin, the position of your finger has to be exactly correct - the difference between a natural and a sharp or flat is about the width of your finger (perhaps a little less) and the difference between a note and a nasty-sounding near miss is significantly less than that. Beginners often stick small coloured spots t the fingerboard to help them find the right positions, but then you have to spend time looking at your fingers and not at the sheet music, which limits you to simple tunes so you can remember where you are. Again, this sounds boring, but it's something you'll need to figure out in order to play anything fun. Find some exercises; there's plenty of simple tunes designed to hop between a handful of notes to help you fix in your mind the correct place for your fingers.
Up to this point, it sounds like these are all things that you can learn on your own, and to a certain extent you can. I had practice recordings (on cassette tape, back then; these days they're on iTunes and YouTube...) that will let you hear the correct notes and timing so you can self-correct. However, there are a lot of things that really require you to have a teacher to watch and correct. As with many skills, practising for too long without a knowledgeable teacher will only cement bad habits. I went a year or so without having any violin lessons so that I could focus on my studies at school, but during that time I continued playing several times a week, on my own and in an orchestra, so I figured I'd keep my skills up to scratch. When I went back to lessons, I was surprised to find that the tutor was pointing out all sorts of bad habits and technique issues, some of which were things that I used to do properly and had simply started to slip on. Some of these things seemed small and unimportant, so I was initially annoyed that the tutor was focusing on them, but very quickly I started to see that they tied into other things that I was only starting to learn. For example, what I thought was a silly nitpick about how I held my wrist and thumb turned out to be very important in allowing me to use vibrato without ending up with a strained wrist or seriously disturbing the way I held the violin.
It's at this point that it becomes apparent that violin has a great deal of interconnected aspects. The way you hold the bow affects your ability to apply the right pressure and to make sure it contacts the strings in the right place; your elbow position affects whether you can change strings fluidly and control your bowing speed, your wrist affects how smoothly your bow will change direction and your other wrist affects how well your fingers can move around the fingerboard. Your chin, neck, and shoulder help to support the weight so that your arm is free to move up and down to allow your fingers to reach the notes, your back and general posture allow you to keep your arms, head, and neck in that position for extended periods of time, and if you're playing standing up, then the position of your feet helps you to keep your balance and support that posture comfortably.
Once you have the correct posture, you have to think about the speed, position, angle, and pressure of your bow, the position and pressure of your fingers on the fingerboard, and occasionally other things like whether you're bowing an upstroke or a downstroke (which is sometimes important) or whether you're shifting position (where you slide your left hand so that the first finger is in the position usually occupied by the third, or a hypothetical fifth, or seventh...).
So then, while continuously monitoring yourself for the correct position of your feet, back, chin, left shoulder, both elbows, both wrists, all your fingers and thumbs; and controlling your bow's speed, angle, position, pressure, and direction; and while checking your the position of your left hand and fingers... then you can start thinking about the music itself; its time signature, key signature, actually reading the music and working out which notes correspond to which strings and finger positions and whether you need to be sliding up or down to a different position... There's a lot going on, and unlike some other instruments, with a violin most of those things are required - if you're not doing all of those things and getting the majority of them correct, then at best you're going to sound like an unskilled player; at worst, you'll sound pretty awful.
Having said all that, I would encourage anyone who feels put off by my answer so far to go and listen to a few beautiful violin pieces and remind themselves that with effort and dedication, you'll end up being able to play what is, in my (probably somewhat biased) opinion, one of the most versatile, most beautiful, and most thoroughly rewarding instruments I know. Even now, when I pick up the violin after a little time away, or when I try to play a new piece for the first time, there is always a period of time where I sound like a hesitant learner again, until I remember the technique or I become familiar with the piece - but at that point, when you can let your muscle memory and experience handle playing the music and free up a small part of your brain to actually listen to what you're producing... that feeling is one of the best I know.
Yes, it is difficult. Yes, it will take a long time to master. Yes, it is absolutely worth it.
