I just finished installing OSX 10.8 Mountain Lion, and boy was it slow. This is my own fault. I bought my first Mac this year, and I went with the base Mac Mini. I did upgrade the RAM from 2GB to 8GB, but I stuck with the base 500GB 5,400 rpm drive over a faster drive or a solid state drive. I don't regret it, and I would make the same decision again. Nevertheless, the OSX upgrade really drove home the point that the bottleneck in my system is the hard drive.
A friend of mine started the upgrade after I did and finished 20 minutes before me. Mine took almost an hour, but he was done in under 30 minutes. He has a MacBook Air with a slower processor than mine but does have an SSD. Due to his faster drive, he lapped me. For business, I always recommend at least a 7,200 rpm drive if not SSD. 10,000 rpm drives are great, but these days I think it's better to pay more for solid state and smaller capacity rather than pay for larger traditional hard driver. Either way though, your performance investment will pay off.
Think of your billable time. My computer was out of commission for 30 minutes longer than it "needed" to be because of my hardware choices. It's not my only computer, so I had zero down time, but that is not the typical scenario. Most users have one computer at the desk, and if it's down for updates, that is lost productivity and therefore lost money. This is something many law offices fail to understand. Investing in your hardware is just as important as investing in your people.
Let me stress that in day-to-day usage, I am not waiting for my computer. Most of the things I do are not disc-intensive, and the Mac is downright snappy doing them. Even things like file copies and software installations go pretty quickly but are not blazingly fast. That is all fully in line with my expectations; something is always going to be the bottleneck, and in most systems, it's the hard drive. One other critical piece of the puzzle for me is that I do most of my work on a terminal server, so the specifications of my local PC are barely a factor there.
I actually went from a 7,200 rpm drive in my last Windows PC to the 5,400 drive on the Mac, and I did it with my eyes wide open. I wasn't willing to pay a 25% premium to get a 750GB 7,200 rpm drive over the stock 500GB 5,400 rpm drive. There was not even an option to put a smaller 7,200 rpm drive in, which would have been my preference. However, this is probably not the right choice for most people. I'm still happy with my decision, but today served as a huge reminder that the decision had tradeoffs.