I've been a flight sim user since "Aviator" (BBC Micro) and have used just about all of them. Having progressed from FSX to P3D (a user since day one), I gave XP11 a whirl (it was on offer!).
Having played with XP10 a while back, I was prepared [sic] to be under-impressed. Well - XP11 is a different animal altogether... and *deifinitely* worth looking at. Compared to the ancient FSX engine, the lighting is superb - and frame rates are way higher.
The downside is the reluctance to ditch all those lovely add-ons for FSX/P3D... but do you really need *all* of them? Some of the default XP aircraft are high quality - and there are some amazing freeware add-ons as well.
Unless the newly announced MS FS steals the crown, I'd say X-Plane is the way of the future.