WM15 felt like a glorified Raw and WM2K was an over-bloated show with one of the worst endings in Mania history. It was so anticlimactic, I felt like I wasted my money.
That said, neither show is completely devoid of good things.
XV had the Austin/Rock main event which was really good, and a satisfying conclusion to Austin's journey back to the title. The Euro Title match is a bit of a hidden gem in Mania's history with a great character clash between Shane-O-Mac and X-Pac and Triple H's heel turn at the end of that match was the start of his epic rise to the main event.
2000 obviously had the Triangle Ladder Match. This show had way too many mutli man and tag team matches, which really made it feel cluttered and messy, but this match was a groundbreaker that set the bar for the type of Ladder matches we see now while also establishing a new era in Tag Team Wrestling as The Hardyz, the Dudleyz, and Edge/Christian became the high standard for the division during this time. Also, the Jericho/Angle/Benoit two title triple threat was solid. Everything else was forgettable or just garbage.
WM 15 at the time was great. And so were 11,12,13,14 at the time. do they stand the test of time, No pry not. but WM 16/2000 was the worst mania all time. it's really the one of the worst big show cards ever booked especially with all the talent on the roster at the time.