When I started using linux, I started out with a Debian distribution. This was mainly because of the good differentiation between stable, testing and unstable distributions. The useful packaging method and good security support was a bonus. However, it was not easy or practical for all cases.
Because of this, also use Knoppix a lot. This is a practical Debian based distribution which really shines with it’s boot-from-CD feature. Then again, that is also the disadvantage as it was then (a year or two ago, probably better now) difficult to install to harddisk.
Later on, I moved to Ubuntu, and now have both desktop and server Ubuntu installations running. This is a very nice distribution, with a practical and easy installer. It combines many of the advantages of a Debian distribution with the added bonus that the project is more agile, and produces stable versions much more regularly. This really is a system which allows you to switch to a linux desktop quite painlessly.
For me one of the major making points for linux is the speed of the system. When I first started dabbling with linux, I bought a new powerful system use. Having never installed linux before, I at some point needed it quickly and installed Windows first (not a good move at this point in time, an Ubuntu installation finishes much faster than a Windows installation).
Anyway, the end result is that I installed my java development environment and compiled the system I was working on twice. First on a plain Windows, then a plain Ubuntu installation. When compiling from inside IDEA, the linux installation compiled twice as fast. I was stunned as this is amuch bigger advantage than I imagined
A good discussion of Ubuntu linux advantages can be found here.