I have dabbled with Ubuntu for a couple of years now. I installed Lucid Lynx 10.04 on my laptop and 2 desktops 2 years ago and have dual-booted since.
Also, at the office we have a couple of weak old computers that I have set up as Linux-only. My co-workers are dreadfully computer illiterate and don't want to change.
Windows XP + Lucid 10.04 was just dandy and I was very happy all around. I knew that the day would arrive when I had to modernize, and I avoided it until a couple of months ago.
The upgrade was painful and protracted on all but one of the desktops, and made me realize how old-fashioned I am. Now, the new OSs have grown on me a bit, and aside from a handful of HORRIBLE ANNOYANCES that are entirely the fault of arrogant and asinine programmers, I am OK with them. (Crippling Windows Explorer has got to top the list!)
The new UIs are no particular improvement, in my opinion, although I suppose most people think that they look prettier.
I hated Unity a lot more than I hated Aero, but much of that has subsided by now. They are just more cumbersome. To me, the new "concept" of navigation is just more difficult and convoluted than it used to be, in both systems.
One of the older systems simply would not take Ubuntu 12.04. I tried several times, and also Kubuntu and Xubuntu. I even tried re-installing 10.04 and trying to upgrade. I still think that the problem is in the software and not my hardware. It is old but easily meets all the specs.
Otherwise, the upgrades eventually came out fine. My laptop came with Windows 7 Home Premium pre-installed, and I immediately shrunk the partition and made room for an ext4 partition. I upgraded in place from 10.04 to 12.04 because I was a little nervous about diddling with the partitions.
For the desktops, I simply took some older (40GB and 80GB) hard drives and used Gparted and dedicated them to Ubuntu. If you have room in your box, and an old hard drive laying around, that is the cleanest and safest way to do it.
In Precise 12.04, I found that Grub-Customizer works great and will recommend it for dual-booting. In an emergency you can also go into Setup and select your hard drive boot order, if Linux has its own drive.
The only really ugly part was the difficulty of getting my laptop's built in wireless to work in Ubuntu. It took MANY hours of work, just like last time with Lucid. Fortunately, Ubuntu Forums has a group of very nice and helpful moderators. They make it possible for newbies to survive the rigors of starting cold.