Good service. Some issues with e-mail and cancelling.
First time worked really well. Second time there were some mixups with changing the date of tickets. Some of our e-Mails got lost in CHTTS's inbox. We followed their FAQ and cancelled our tickets at the train station and got new ones. All good but the cancel information was not propagated to their system and sorting our refund took a bit of waiting and effort and some e-mailing around. Now everything is in order though and their staff was very helpful and apologetic during the whole process. All in all I recommend their service for booking tickets ahead online. Though you should know that you can buy train tickets cheaper (but cash only) for any train and any date at any china train station.