Ok, Ubuntu Karmic has been out for a few days now, and I have to say that both for me personally, and I get the impression for a good part of the Ubuntu user community, the feelings are slightly mixed. i find this mildly amusing, because when Jaunty came out, six months ago, I thought it was truly excellent, and it was met with a collective yawn, and a lot of comments along the lines of "why bother?". This time, the buildup to the release of Karmic has been pretty significant, fueled to some extent by comparisons with Win7. But now that the release is available, suddenly there is are some complaints...
As I have said previously, one of the biggest changes in Ubuntu is the switch to GRUB 2, at least for new/clean installations. I don't do upgrades on this kind of software, so I have no direct experience with that, but if I understand correctly, installing Karmic as an upgrade to an existing pre-Karmic Ubuntu release will leave Legacy GRUB intact. What I would say is, if you have never done anything or changed anything with GRUB or its configuration files, you're likely to be ok either way. If you have changed GRUB in any way previously, you are likely to be in for a surprise if you make a clean install. Be prepared to read the release documents carefully.
I have seen one other problem so far. When I install on my ASUS N10J, it seems the the installer (Ubiquity) crashes near the end - I think it is during the finall installation and cleanup. The install has progressed far enough that I am left with a functioning system, but certain things are wrong - for one, it will not automatically mount USB disks, and several packages that should have been deleted (such as gparted) are still present. I haven't been able to isolate this far enough to really pin it down, though.
I have also heard from a few people who have other problems, in particular with installing on an HP 2133 Mini-Note. Again, I always make clean installations, and it works just fine for me, but it seems that upgrade installations run into problems with things like sound and wireless networking. Again, be careful.
Last, but certainly not least, the release of Ubuntu is creeping even more in the "commercial" direction, with the addition of things like Ubuntu One, the Software Soure (or whatever it is called), and the likes. If you are a Linux/FOSS "purist", you really need to think very carefully before installing Ubuntu, and look at some of those things.
I have installed Karmic on my Fujitsu S6510 Lifebook, HP 2133 Mini-Note, HP Pavillion dv2-1010ez, and ASUS N10J, with only the few problems mentioned above.
My next few days will be spent looking at UNR 9.10 and the Kubuntu netbook release. I hope to find some nice things to talk about there!