i've heard of people using them. we considered them ourselves. they can get expensive, but still, cheaper than a bank.
i would recommend getting extra supplies like the medium and things like that. some people have tried shipping without it, but it's really not advised. you may see if biotranz will send you extra supplies that way you won't have to buy multiple kits. additionally, if you are doing more than one insemination per cycle, you will need more than one kit. for instance, if you are going to do inseminations on your pre peak day, peak day, and day after that, you would need three kits because sperm would need to be mailed to you 3 times.
you can't store the excess in the fridge and use it the next day - it will die. you need a new sample each time. this is how it can get very expensive very fast, because you need essentially overnight shipping three days in a row, not to mention the price of three kits, and the price of the refillable items (storage container, medium, syringe) for each month times how many times you are inseminating each month.
additionally, your KD must follow the directions to the T for the mailer. all the instructions are important regarding the rest time and what not.