Was Windows 98 the Pinnacle Microsoft Operating Systems?

I have recently loaded a copy of Win98 on to and old PIII machine and it flys. I’m running much higher spec machines with XP and Vista and these seem to run much slower that a PIII running Win98. This leaves me wondering if it’s worth upgrading your machine and PC. Is there really something so compelling about the latest operating systems and hardware that make upgrade worth the cost.