I was likely one of their first batch of users in 2006 and have been using the Pro version since then. At the time I was traveling a lot for my work and I thought it was the greatest thing since sliced bread. I still think that. I have close to 300 trips in there and have many times gone back to look at a trip I made a few years ago to remember that great hotel (etc) where we stayed.
The Pro version has always been worth the modest amount they charge. There were many years when it notified me of flight/gate changes or delays before United did. There are other features I haven't kept track of, but they been adding very useful features steadily. The best of these is the way it handles confirmations that are forwarded to them (really, all I do is hit the forward button; no need to edit them in any way). It always handled the main ones like airlines, hotels etc. but the ones that weren't in a main system of some kind were put in "unfiled" and we needed to enter those by hand. Now almost all of them go straight into the plans.
My only complaint/wish is that they would provide a way to download all of our trip data to our own hard drives. I have lived in some fear that they will be bought out, or some other change, and all of our data (considerable and irreplaceable for me) will be unavailable. They also haven't updated some of their interface in years, but the app itself just gets better and better. Highly recommended!