I was happy with Tiger on my original 2005 Mini...... It did what I wanted and being my first computer I didn't know any better.
When the HDD and the power supply failed early in 2009 I decided on a new Mini rather than a repair. For the short time I had Leopard I was a bit annoyed with some of its shortcomings. All was sweet when Snow Leopard came and I installed it.
I didn't bother with Lion. I coveted the Dictionary app upgrade, which i saw on a friend's machine. Otherwise I was none the wiser, having used only a cricket scoring program running under the Windows environment, using Parallels.
By mid 2012 the Mini was getting slow. I guessed it was short of RAM for updated apps, so I increased the RAM to 5 GB (from 1 GB), and upgraded directly to Mountain Lion. In general I am happy with the upgrades.
I did get the 10.8.2 update, that was later pulled.
One thing that does not seem to be right, however, is Time Machine. Since Mountain Lion was installed seems to given to doing massive, multiple GB, back-ups, which take ages to do. It keeps the CPU very busy, and raises temperature and fan speeds quite a bit.
Previously Time Machine back-ups were usually only a few kilobytes or MB. After nearly three years I had over 270 GB of a 320 GB back-up disc free. Now, after about 3 months I am down to 17 GB free.