There can't be any real debate over the underlying kernel and subsystems that make up Mac OS X. They blow away earlier Mac operating systems with their capabilities.
Their are two real criticisms that you can make, though. First, OS X does not retain the best elements of OS 9. It breaks many of the conventions that Mac users have grown used to over the last 10 or more years, and does not replace many of them with a better alternative. Most Mac users do not know Unix, but they have intimate experience with the Mac user interface, and love it.
A great discussion of the interface problems with OS X can be found at Ars Technica:
The other valid criticism is that OS X does not offer the business/enterprise features that other operating systems do, and that there is no real business support. Out of the box, most Unix/Linux distributions support a slew of Enterprise services. Likewise, Windows 2000 supports all every business service you might want. Microsoft has excellent business support, and there are many companies supporting Unix/Linux for business (ie., IBM). Business support for Mac OS's has never been there, with the result that only businesses without complex requirements have ever taken Mac seriously.
This doesn't mean that Mac OS X can't do these things. Any good Unix guru should be able to get the important Unix software packages to compile and run on it. Most admins' time is too valuable, though, to be experimenting.
OS X should mature to be an interesting new client operating system. It is good enough to eliminate the likelihood of Linux ever being a significant desktop presence. On the other hand, it won't offer any significant alternative for enterprise servers.