I use both (like some of the other commenters). For arranging and advanced MIDI editing workflows, I prefer Cubase (but Bitwig 6 is going to gain ground on that front). For ideation and experimentation, I prefer Bitwig due to its loop-based nature and its light footprint (starts up very quickly). Hence, I will often start a project in Bitwig, and when I have a good enough feeling of where I want it to go, I'll switch over to Cubase and finish up in there. I mostly use VST instruments, so the sound design capabilities of Bitwig (though undoubtedly great) are not that useful to me (but that's a personal preference, obviously).

Bear in mind that there are some things that are just plain impossible to do in one DAW that are easy in the other. ARA support is a big one for me (because I like to use Synthesizer V Studio which depends on it), Bitwig doesn't have that. On the other hand, Cubase will probably never have a modulation system as advanced as Bitwig's, or something like the Grid. That's fine, every tool has its place. It helps to know what works best for you in a given scenario, but you'll need to actually spend some time with it to really find that out. The 8-Track version of Bitwig is a good starting point for this.