To 99% of people it's overkill. The £/performance ratio is too much. SLI is almost never supported properly for the latest and most popular games. It's really only for the extremely wealthy.
A single 1080Ti does 4k60 for 99% of modern games (the remaining 1% being unoptimized nvidia-only gimmicks, like ultra hairworks in Witcher 3, or HBAO+ in RE7)
Its nice not having to mess and tweak with dual cards nowadays. It was always a pain with CrossFire when new games came out or obscure small games.
Fantastic when it worked though. Battlefield 3 and 4 on 64 player servers. So smooth.