We are traveling around Asia and have already tried bullet trains in Japan and South Korea, where comfort and service are both excellent (especially in Japan).
In China, we've taken six long bullet train rides. It's efficient, very fast (faster than in Japan and South Korea), and compared to airport waiting times, high-speed trains are a much better alternative.
On AliPay (which is essential in China), you can book and pay for tickets 14 days before departure. It's a great system. The only issue is that you might not get seats next to your travel partner - you often have to swap with other passengers.
You can buy tickets for 3 classes (second class, business, and first class), and it's not cheap. We traveled in second class and were quite satisfied: there's more space and freedom to move around compared to a plane.
Much of the communication - both at the stations and on the trains - is in both Mandarin and English, so don't worry about figuring out where to get on and off the train. It's actually quite easy.
On particularly popular routes, the trains are quite full, so you need to be quick to secure space for your luggage in the very small compartments. If you can't, you'll have to keep your suitcase or backpack between your legs during the trip. Therefore: Arrive at the station early and make sure to get to the front of the queue.
You can buy food and drinks on the trains, but it's expensive and not very good. We got used to bringing our own supplies, which can be bought at the stations. It's a good idea to buy fruit before you get to the station.
The toilets are decent; there are both Western-style and squat toilets.