I would also like to chime in with my experience, as it seems that I hardly ever see anyone writing about good things.
I am on an early 2007 MacBook Pro, I upgraded to Leopard within a week of it's release. I've had a few hiccups, but never anything major. Airport works well, I have an Airport Extreme as my wireless hub. I don't have any issues with it, and I used to have ichat running all night, sometimes for 12 hours at a time, without interuption. I don't have any special features or plugins etc installed. TimeMachine works for me now, at first I had issues because I have a USB HDD attached to the AP Extreme and had it setup incorrectly. But, once I figured out what I was doing, it's worked ever since. I run firefox, which works well. I've run Safari, and never noticed an issue, but I don't run it much. I use Office, Photoshop, Lightroom, Itunes and Firefox mostly. Do I have crashes? Yes, but I don't remember with what or when...which means they are so infrequent and not with anything specific. So, I'd say I have maybe 10% of the issues with my Mac that I do with the Windows PC I have at work.
My biggest issue is I have a dead battery, with only 129 cycles. It's a hardware issue, not software.
All that aside, I'm much happier with my Mac than I was with a Windows PC. It's simple, clean, and does exactly what I want. I have used Windows back to 3.11, I used to work as a Tech Support for the library computers (both public and the librarians personal PCs) of a large university, I've build numerous computers myself, installed Windows of all variations, Many different Linux packages, BeOS 4 and 5 and a couple other runt OSes here and there. I just like my Mac and want people reading to know, that not everyone has the issues many people complain about in here. And to those who are complaining...why don't you use a different OS? Go back to Tiger. Go to Windows, Linux, whatever. I know you spent the money on the OS...I'm sorry for that. I don't know what you can do about that, maybe sell your license to someone else.
A very happy Leopard and Mac user in general.