OS X is by and large a good OS, but once you get past the sexy UI you find a lot of rough edges.
For example, this month I've been working remotely over a flakey DSL connection. I ran into a very frustrating problem: if you're using a PPTP-based VPN, and your network connection is poor quality, the whole Apple UI will frequently freeze up with the "Spinning beachball" cursor for minutes at a time.
Luckily for me the work-around is to reboot my DSL modem. But it seems like poor system design for the VPN packet performance to affect the UI of non-networked applications.