I bought a Mopho x4 recently, with the goal of replacing some Virus tracks. After dialing in the same patches, I realized that the Virus does a great job of emulating analog, I mean they're practically indistinguishable.
So I decided to set them both as similar and simple as possible to see how the Virus (kB) stacks up. One saw-tooth oscillator, no detune, same Amp envelopes, really as similar as possible. When I hit the same middle C on both synths, they were exactly the same, I mean dead-on.
As the Virus went up in pitch (higher up the keyboard) it lost some "density", which was brought back with a small amount of Analog Boost (under 20). When the Virus went down in pitch (lower on the keyboard) it was "thicker" than the Mopho, like a heavier low-mids. Unfortunately on the kB there is no EQ, so there was no compensating for the "low boost". So, as far as actually doing a side-by-side comparison of the Virus and an analog, it was an even match (at middle C, with a basic saw-tooth). But this is just one particular analog, and one specific waveform, and not considering the filters (which were noticeably different, the Virus being more "controlled").
When it comes to Digital vs. Analog, what really needs to be said is this: use what works best for you. The Mopho does indeed have some great tones, possibly unobtainable with the Virus. The Virus certainly has some great tones, certainly unobtainable with the Mopho. But they both serve a useful purpose in my studio, and live.
If I were to play a song containing both the Virus and the Mopho for someone, and ask, "Which tracks are analog?" I don't think they'd be able to tell. The Virus plays analog very well. But it also plays digital very well, and can make patches that are undeniably digital sounding that my Mopho couldn't dream of.
But when it all comes down to it, I'm glad I have both. If I want the Virus to sound analog, easy enough. But now I don't feel that I need to, and this leaves the door open for a wider variety of sounds overall.