zerozero's instructions worked fine for my Thinkpad R60 laptop. No problems with KDE 4.10.1 under Linux Mint Nadia KDE. Earlier I had already upgraded the kernel to 3.8.4 just for fun, so dist-upgrade did not touch the kernel, which is a good thing. Thanks, zerozero!
Initial impressions--"Con" department--boot-time seems slower by about a few seconds. Not a big deal, and some of that is probably due to inevitable file fragmentation. Initially, KDE came up with a password logon window, which it shouldn't, because my system is configured to auto login, but I think I allowed the upgrade to overwrite some part of my config, so I just went into Settings Manager, poked around until I found the right setting ("Login Screen" under System Administration in the System Settings) and set it back to auto logon, without the password, like I prefer, being a lazy risk-taker.
Now the system is back to the way I like it and everything has settled down.
Pro--I do believe that Dolphin, the file manager, seems faster both starting up and navigating through folders. Much faster starting up. I'm not sure whether Firefox is any faster, but it certainly isn't slower. In fact, overall the system does seem a bit more responsive, although 4.9.5 wasn't exactly slow on my Intel hardware. My Intel laptop gives better much performance than my AMD/ATI desktop. Intel just works better with Linux all the way around. I did not notice any change in the icons, but then I am using Oxygen I think and a dark black background, so that might explain that.
Is the new version worth upgrading? Depends on whether one has upgraditis, although I think I will definitely upgrade my KDE 4.9.5 AMD/ATI desktop now because it does seem sluggish. The desktop uses an AMD gpu, and it may be that the new version of KDE has improved things were AMD/ATI is concerned, but I don't know. I think most people should probably wait until Linux Mint 15 KDE, but definitely people who use Dolphin a lot should probably go for it. I don't have many widgets, so I can't speak about the performance there.
I think upgrading Ktorrent to 4.3.1 had an even bigger payoff, although that process was a lot more involved than upgrading KDE.
Of course, in the last 48 hours, 4.10.2 has been released, but has not made its way to Kubuntu's backports repository yet. At a certain point one has to weigh the advantage of upgrading versus the advantage of being lazy and waiting a while.
Update - Thursday, April 04, 2013
Upgraded Linux Mint Nadia KDE to 4.10.1 on my AMD/ATI desktop (an Asus E350M1-M with an AMD-E350 apu) using zerozero's method. This time the installation was completely unattended--I entered the upgrade command, then went to work, and the thing was just finishing when I came back. I think it paused at the prompt where it asked whether I wanted to overwrite the config file. When I moved the mouse, it defaulted to "N"o, do not overwrite the config file, and finished the upgrade. So I can say that it is not necessary to overwrite the config file. Save time and just keep your config.
Everything peachy - using Kernel 3.8.5 on the desktop too - KDE 4.10.1