I’m on a late-2009 MBP (like @daneel) and the upgrade is good. There were some issues along the way with some of the other OS X versions (10.7 especially) - inexplicable poor performance, apparently broken stuff - and it had me so frustrated at times that I considered going back to Snow Leopard (which is what my MBP came with).
But all the problems I had steadily disappeared, and have been gone entirely since whatever the last one was (10.8), and this one (10.9) is supposed to offer significant performance improvements under the hood (the stuff daneel mentions is truly worthless, but there’s much more to it than that), which seem to be true in my limited testing. It’s worth it for users of older computers to upgrade.
The fact is I think Apple does still care about OS X, because though it’s taken a while, in its current state they’ve gotten almost everything right. And when there are no obvious big features to work on (not that there aren’t a lot of things they could do), they put a lot of effort into behind the scenes stuff, which have improved dramatically in many different ways since 10.5.
I’m really not a fanboy but I don’t want to use any other OS (I came to OS X from Linux) and I don’t want them to make major changes without slowly introducing them (which is what they do), because it works great as it is. And I think they know that, and they’re careful with it, and they clearly try to ensure older machines can run the latest versions - the upgrades are dead simple and work without a hitch.
With mobile devices they can’t do that because the underlying stuff is changing a lot more quickly. My Nexus One simply can’t run the latest couple versions of Android, and I don’t complain about that… it’s beyond obsolete. So are early iPhones and the original iPad, though since Apple kept the interface so consistent (unlike on Android), at least until the latest iOS, many people don’t realize that.
