There's alot of good advice on here, so here's my two bits worth. I've spent quite alot of time in freeform sparring with friends who practice Jeet Kune Do type systems and pure Shotokan Karate. Both of these guys use alot more kicks than I do (I view them as flash if you have time but generally anything above knee height is too slow - could be my legs are slow but neither Tai Chi nor Wing Chun advise kicking high.) I have found that iriminage techniques work extremely well against karate style snap and push kicks, entering around the kick and executing your technique as your partner hurriedly tries to get his/her foot back on the floor. I do employ waist high front kicks quite alot as they are good for seting an opponent up for follow up hand techniques or just to push him/her out of reach. The only kick I had serious trouble getting around was the Thai shin kick that my JKD friend favours. I'd get inside the shin and get a knee in my back - and he's not afraid of going to the floor either so the resulting crash of limbs to the mat gets pretty messy.
Kicks are extremely powerful if you can land them and are best employed (as was shown in a previous post) to take care of secondary opponents at range or against relatively unskilled attackers.