I actually moved from Linux to a Mac not too long ago. I bought the Mini 2.40GHz and put 8GB of RAM in it. I've never run Lightroom on any OS before so I have nothing to compare it to, but here are my impressions:
I've got the K-5 and process raw with Lightroom and Photoshop. I have Lightroom render minimal previews for the raw files on import. It takes a few seconds for a photo to load for view and a few seconds more to generate a 1:1. I think I read you can have Lightroom pre-render some of these views but of course that slows down the import. At any rate what I'm experiencing wouldn't be considered "snappy."
There have been times where I've used the adjustment brush extensively on a photo where the editing process becomes stop and go. There just isn't enough horsepower available. So you sit waiting. If you do some research online, you'll find others have had this problem with Lightroom. I've stopped using the adjustment brush, except for small things, in Lightroom. If I want to selective sharpen or underexpose an area, I now do that in Photoshop, which handles these tasks with ease.
You asked for "in general" impressions about OS X. As I mentioned I'm a Linux user and have been since about 1990. I've also used Windows 95 through 7. Moving from those operating systems to a Mac isn't a huge deal. There are some basic things like installing programs, keyboard shortcuts, window handling, etc. that are different and must be learned. It really comes down to personal preference and the only way to discover *your* preference is to use the OS. But if you're reasonably computer savvy, you shouldn't have any problems. I moved from my Linux workstation simply because I wanted to run Adobe products and OS X runs on a Unix base. So being a command line type guy I naturally gravitated to a Mac. I suspect the next one I purchase will have more horsepower though.