One of the problems with recommending Ubuntu to potential future users, especially those not particularly given to technical endeavours, is that there is a chance that upgrades will break their machine, and they'll have to pay or otherwise coerce some knowledgeable person into fixing them.
In my limited experience of running successive versions of Ubuntu since 8-something on a couple of different laptops, this chance is quite high. I'm not sure if I'm just unlucky with the hardware that I'm using, or if it's a result of the higher-than-average number of packages I have installed, or if upgrades are just typically problematic.
So I'd like to know the likelihood, for a casual user, of doing a release upgrade, for example from 10.04 to 10.10, without experiencing any regression bugs.
Obviously this is dependent on the hardware that people are running. Canonical seems to be making some efforts towards collecting data on this, for example with the "I am affected by this bug" checkbox on their issue tracker, and with the laptop compatibility reports, but I've not seen anything comprehensive.
I'm hoping for an objective reference here, for example a study carried out by relatively unbiased individuals. However, anecdotal evidence is probably useful too.
Best Answer
I'd love for someone to come up with some more numbers on this, here's my little analysis:
There are around 100 bugs tagged "regression-release", that are of "High" importance. A further three are marked "critical", two of those affect a bunch of specific ThinkPad models.
Have a look at the regression tracker, note only the ones that are tagged "regression-release".
There are quite a lot of them, of course, but then the average bug affects only a few users (now that's a number I'd like to know). Note that pretty much all of the 'serious' bugs have to do with some specific piece of hardware, and thus wouldn't affect "well supported" hardware platforms.
Take the time to sort this list by importance and read some of the descriptions. To me at least, many of them appear very minor, or affect only a small set of users. But I can't be the judge of that.
The likelihood of a normal user experiencing a release upgrade regression is almost impossible to estimate. The hardware platforms vary immensely.
In my experience, everything works perfectly all the time. See how worthless that information is. ;-)
I fear that's all I've got. It's really not a problem that is discussed much, which is why I have to doubt it exists at all. People usually only investigate such things in detail, create comprehensive statistical analysis, if many users are affected; due to the way Canonical and the community test before they release, this appears to be rare.