I've recently been spending a whole bunch of time researching and playing around with different encounter building tools. Yours is one that I really liked. I liked how it shows both the encounter cost and the XP award (some others show only one of those, or the other, but not both).
I also really liked how it shows difficulty of the encounter as a scale, rather than just declaring "this is moderate" or "this is severe". That was particularly annoying because it seems like there's an inconsistency with how other sites decide which category it fits into. Most sites seemed to think that 81 XP makes a severe encounter, because 80 XP is the limit for a medium encounter. Some said 80 XP is where a medium encounter starts and all the way up to 119 is medium. Intuitively I feel like the latter is how it should work, but most seem to disagree. By displaying it on a sliding visual scale like you do, you sidestep the problem and allow me to come to my own conclusion.
Personally I'm not likely to actually use it all that much, purely because the Pathbuilder encounter builder also lets me save and come back to multiple different encounters while I'm planning out a campaign. Even better, it includes initiative tracking with optional integration to players' character sheets. For purely encounter building, yours is a much nicer user experience, but Pathbuilder fits into a whole ecosystem a bit more nicely.
