I suppose I've done over half my studies remotely by distance learning. For some people, it's the ideal way to go, but there are a lot of pros and cons. It depends to a large extent on each individual whether it works best for them.
For a start, you should never underestimate the commitment and self-discipline you need to study on your own. Usually you get some support in your learning from the college, but it's never the same as attending class, meeting your teacher face to face and interacting with other students. Though when you don't have the money or time to go for full-time education, or the course or college you want to go to is too far away, then it can be a practical solution.
My advice to students who want to study by distance learning is to set goals, set a timetable and keep to this as far as possible. Studying on your own often requires quite a lot of stamina and it should become second nature – and as pleasurable as possible. So leaving things to the last minute and juggling with all the other demands on your time have to be kept to a minimum. If you don't have the luxury of having time and space to yourself at home, then maybe you could do like I did, go to your office early or stay late to study, so that family life doesn't get in the way of your studies.
Some subjects are easier than others to learn in this way. Learning a language is not always as easy as subjects that rely less on interacting with others. Here it is useful to have contact with a teacher. Languages have that unique feature which depends on interaction between people. That said though, there's still a lot you can do from a distance too (like being an active member on a forum of like-minded people).
Self-discipline is essential as there's no one immediately on hand to push you. Not having classmates to discuss and share problems with can be a disadvantage too, depending on the course, and costs, you might be assigned a tutor responsible for giving you work, but this will vary from course to course. One distance learning course I did much more recently was very good in this way, with an assigned tutor giving me lots of very useful feedback almost instantly. The continuous contact meant I was very motivated to complete the assignments efficiently and on time. In the middle of this course I was sent to work for six months abroad and whilst this was a bit of a disruption to my routine, I was still able to continue as it doesn't matter where you are (I was even able to write one of my assignments about French management systems, since this was where I was working and it all was very relevant to me at the time). Though in theory it might have been possible to finish the course in 18 months, I took nearer to two years for me to complete it, but most distance learning courses tend to offer some flexibility. Personally, I think it's best not to let it drag on over too long a period of time, as then motivation can drop.
Usually juggling work, home and study, along with good time management is the key to success. When studying a 'real' distance learning masters' degree over two years, I used to dedicate at least one whole day at the weekend every week plus time every day during the week to keep on top of the work I need to do. I got into a routine of going to my office very early before my colleagues came in. I'd go through course material an hour or so before starting work. Then time was taken at the weekends to research and complete the many assignments that came with the course (I was churning out an assignment for one or other course every fortnight). One day per year we were expected to spend the whole day with our classmates and meet the tutors at the college, which was located outside of London, not very far from where I was living. Not having to travel to class was a definite advantage and saved a lot of time and expense. I think some students kept much more in contact with each other than I did, though I actually preferred studying independently. Of course this means you don't get much of a chance to discuss or compare notes with the other students, and there's no social side to learning. If you can keep in contact with fellow distance-learning students, this might help. I suppose this depends how much you like social networking.
Depending on the course though, it can be a huge undertaking. It can be very lonely. Though full-time, my first masters' degree was in fact a full-time course, it was totally by research and I used to see my tutor for an hour every fortnight. The rest of the time was spent pored over documents and conducting research on a very specific area of history in a city several hours' drive from the university where I was studying. There were no classes, no one else was studying the area I was looking at and I used to generally get up every morning at the same time, go to the library and work all day. I did this for twelve months (with about four weeks' holiday). It was 'distant' in the sense I was on my own, away from any classroom, though I was in the lucky position of having all my time free to study. Most distant learning courses however, are not as simple as this.
One of the biggest advantages of distance learning is that you can fit it around your own schedule, studying when you have the time and usually at your own pace. For people who can't take time off from work, this offers the opportunity to gain further qualifications or do something that interests them.