We don't have a Diplomat, but have a full wall slide on a 34' Monarch. The slide itself is 25' long, from behind the driver's seat to just in front of the bedroom rear wall. As with DipHop, we love the open feeling front to back and the full wall slide was the primary reason we bought the unit.
Also as with DipHop, our only problem has been with the slide topper. Early on we made the mistake of driving in high winds
(60-70 mph gusts on I-10 near Winslow Arizona) and the slide topper billowed out and ripped about six feet from the front edge before we realized what was happening. It seems the design could be changed to provide a windbreak for the front edge when the slide is retracted to prevent billowing and ripping. Since then we've been careful - no driving in high winds and we bring the slide in when camped in high winds (Mojave Desert gets windy).
We also had the amusing experience of a bird building a nest under the slide topper while camping in Las Vegas. I suppose it could happen on any slide, but with a 25' slide and a bird-nest right in the middle, it was a little challenging to get the nest and debris out to be able to retract without bringing the nest inside! A 40' slide would be even more of a challenge.
We are careful about maintenance: lubing the hydraulic pistons that move the slide and watching the tank of fluid, and lubing the slide seals.
Overall we're very happy with the full-wall slide and would recommend it