I've had DCS since about 2009 or so. I bought my first DCS/PS2 engine around 2008 (I think) and knew that it had the capabilities. I had been reading about it since it was introduced - but wasn't into 3-rail all that much at the time. When I got that first engine, the TIUs and Remotes were hard to come by. I actually had to buy them separately from two different dealers because no one had the combo set.
Initially I used it to run a very unrealistic mix of 3-rail and postwar A.C. Gilbert era American Flyer S-gauge under the Christmas tree each year. I liked that I could use a separate channel (variable) to run the AF stuff rom the remote. I still do this from time to time. Now that I'm building a permanent layout in my basement, it will be at the heart of the electrical system there.
DCS is truly well engineered. I know there are issues with the early 5V boards, but knocking on wood I've not had any issues with the one used locomotive I have with a 5V board. The other PS2 3V and PS3 engines have been great. Probably the best aspect of the DCS protocol compared to TMCC/Legacy is two-way communications. This ensures that the engine receives the command and positively responds to it. TMCC and Legacy are simpler and also good, but when operating TMCC or Legacy locos in TMCC mode, there are times when a command is missed. This is particularly annoying when running two or more locomotives together.
I've found that DCS is not quite as good at very slow speed operations compared to TMCC or Legacy locos in 100 or 200 speed step mode, but are much superior to TMCC 32 speed step mode. However, an operating tip is that with DCS, if you get the throttle up to 3 or 4 MPH and then feather the throttle back down to 1 or 2, you can achieve more realistic slow speed operations. The ability to adjust the acceleration and deceleration is also a very cool feature.
Another thing I really like is the ability to record and playback an operation session. I find this useful when I want to have my trains running in the background and "do other things" in my shop.
However, the feature that takes the cake for a nerd like me is the user re-programmability. I've customized the sounds in multiple locomotives, and not having to buy a whole new board - or ship it back to the manufacture when something is wrong - is great!