Having been an Apple customer since the Apple IIc, I was worried. OS 9 had a snappiness to it. OS X would struggle to change windows. At the time, Windows 98 and even moreso Windows NT could run circles around OS X. With XP dropping the same year, Windows consumers were running a really well optimized OS compared to an OS X that needed a lot of work.
Apple has come a long way since those dark times to having a truly enviable OS. They quickly got 10.1 out the door and 10.2 and 10.3 shipped in around 1-year increments (with 10.4 shipping a year and a half after 10.3). Apple really did a lot of work and, at this point, I don't think anyone questions Apple's ability to push a great OS, but there were some really bad times.
I remember people questioning whether it was that the Mach kernel was just inherently slow (igniting lots of micro vs monolithic debates) or whether the compositing window manager was worth anything or coming up with theories that it was because Apple replaced Display PostScript with Display PDF to get around Adobe licensing or whatnot. Today, the majority of Mac users probably never experienced that bad time of trepidation when the future of the Mac platform was in doubt. Today, it's really great to be a Mac user. It's hard to imagine a time when Apple's products were so fringe. At nearly 10% of the market (and nearly all of the premium market), Apple feels so strong and people really know Apple (both for computers and other devices).
Kudos to Apple for taking the risk and getting through the bad transition to what is a really wonderful OS. And kudos for making each new OS version feel better than the previous.