You don't need very many samples to get a good statistical analysis. Is this one, no, but we only need like 79 from a randomly selected bunch who have purchased the game.
That said, this thread supports the fact that most people didn't really/don't really play Doom 3. We also have no idea how this engine will scale to outdoor environments, etc. What Doom 3 has given us, in essence, is another 3dMark at this point. A look at possible future implementations of today's technology. I think that the D3 engine will have to be very heavily tweaked based on what I have read about Quake4. So using Doom 3 for a benchmark right now is kind of like using 3dMark03 was for DX9 performance. Nvidia's cards didn't show to bad in that, but when we actually saw real implemenation of DX9, the FX series was shown to be far inferior in shading performance. My guess is that Quake4 could be similar, with ATI's Xxxx cards looking really terrible compared to the 6xxx series. But that is all it is, a guess, and that is why Doom 3 performance is nearly a synthetic benchmark at this point.
Games may be highly subjective, but Doom 3 seems to be widely looked upon as a gameplay failure, just reading a lot of posts and remembering the disapointment following its release by many people across many forums. I am glad that everyone who enjoys it really likes it, when looking for a good flashlight mod I ran across several forums that had several people who absolutely loved the darkness, the zombies, the demons. Cool for all of you guys, but frankly I am still amazed that it continues to sell like it does. id + aggressive marketing = a sales juggernaut, evidently...
Source engine performance is just much more important, right now. HL2 is not only arguably the best single player game currently available on the PC, some may feel different, but I said this was an arguable point and everyone that I know has played got a lot of satisfaction out of different aspects of the game. The gravity gun just rocks. But beyond that, how about CS:Source? How many thousands of people are playing that right now? If I were at home I could look at the metrics and see 🙂 What really sucks about it is how Valve keeps breaking the benchmarks that are available everytime they feel the need to tweak the game. At least they fixed the Video Stress Test in one of the last updates... but performance for this game is very important when making decisions now about what video card to buy.
What I am trying to say out of all of this is that we can really get much out of the D3 time demos and rely on them to tell us how we will be able to play future games that run on that engine. We can get some inkling, but it may be very far off. Lets hope that video cards that can easily play D3 will do the same for future titles, but as for me I am not holding my breath. And to top that off, it may turn out that those games we are anticipating suck. Raven is doing Quake4, and I have hated every game they have ever made. They haven't been able to do level design to save their ass, swinging a light saber only makes up for that to a certain degree. I am stoked about Q4, and Prey, but if they look and play like D3, I won't bother to pick them up 🙂