For what looked like a really easy question, I really don't have much of an answer.
I had wanted to try it for several years but never did. One summer myself and another friend decided we were going to learn to tele that next season so we bought some used gear off of the 'net and just went out the first day the next season. No lessons, no nothing, just two idiots saying "Well I've seen other people do it before so I'll just do what they did". Actually it worked out pretty well, we were skiing blues that first day and laughing our asses off at each other falling all the time. It helped that both of us had 20+ years alpine experience and could always bail to parallel skiing if we needed to. That first season it was so different to be challenged again at the local areas, it opened up new frontiers.
I went both ways (alpine & tele) for the first 3 seasons. I've still got the alpine equipment, but I've only been on it twice in the last 3 seasons. I love telemarking but I really can't tell you why. My buddy, has gone in the opposite direction though. He hasn't been on his teles in the last 3 seasons, originally because he was coming back from an injury and teaching his son how to ski but even without those as excuses he hasn't come back to tele. So, obviously it isn't the be all and end all for everyone who tries it. I also spent a couple of seaons learning to board (fairly well) but that just didn't hold my interest, so its not just that tele was something different.
I guess I originally started because it was getting old blowing down my local hill in 30 seconds and being able to ski the most challenging line I could find without really trying. In the Southeast I think that is why most of the skiers start telemarking, the short, easy slopes are conducive to making you want to try something new. Tele skiing is more physically taxing (especially intitially) than alpine and I still (after 5 seasons) have a lot of work to do on my skills even though I am comfortable skiing anything out West on my teles that is short of the insanity level. Double blacks at Snowbird push the envelope for me, and anything with a mandatory air involving either high consequences from a fall or greater than 5-6' drop are out of the question.
There's just something about the feeling of carving that teleturn, or being out on a not so deep powder day when everyone else is shin deep but you get to feel the pow against your thighs, or a deeper day when you disappear into the snow on every turn when everyone else is just getting face shots. If I were really a writer I could find better words to explain all the sensations and emotions involved; instead I'm just a simple tele-skier. So you'll just have to live with this explanation; telemarking has soul for me. You'll know after you try it if it holds the same for you.