I've moved back and forth over the last couple of decade or so between Macs, Windows, Slackware, the BSD's, and more recently Ubuntu. I chose it basically because:
- I'd heard good things
- I had some CD's handy and no decent internet connection available at the time
- I needed a Linux to support some annoying hardware.
It served me well for a couple of years.
My eventual reason for switching was due to my personal preferences, and it's gradual shift further away from them:
I'm prone to minimalism, and I found that Ubuntu was always doing things automagically that I didn't want behind my back that took an inordinate amount of work to get rid of - sometimes with multiple, conflicting packages modifying each others behaviour. Particularly around upgrade time, it was just a PITA to figure out what needed changing all over again to set things back to how I liked them.
A lot of their work goes into features that I really just have no use for - PulseAudio was unpleasant, and offered me no benefits over ALSA. HAL and DBUS were both irrelevant AND annoying to the way I like to use my computer. I don't use GNOME, or KDE, so improvements in those areas just weren't interesting to me.
The final straw was an update that meant compiz just wouldn't run anymore on my hardware - which, coming after several days of customizing it, and it being a laptop with no chance of upgrading, was a Big Deal to me.
So I've been running arch for nearly a year now, and am fairly happy with it. In all honesty, I'd probably prefer to go back to FreeBSD, but with no pressing need, I find these days that I'm not really willing to invest the time to change - it's not broken, and I'd rather spend my time doing other things.
I've tweaked Arch into a state that I'm fairly happy with, and their design decisions generally mesh well with my way of doing things.
Fundamentally, for me, it was matter of personal taste - and my taste isn't really Ubuntu. It was definitely worth running such a variety - I now have a fairly good idea of how they differ, and more importantly to me, of what I want out of a system.