On the whole, the Sigma CTS-V is one of the best cars GM had produced in decades up to that point. The performance of the vehicle was in fact world-class, and ultimately, Cadillac won a lot of praise for it. The CTS itself, however, is a bit more of a touch-and-go story. Admittedly the interior isn't as pleasant as your comparable BMW or Audi, but then again, for the same price you aren't getting a 400 BHP rocket. The case for the CTS was the same as many GM vehicles of that era, getting better every single year until the end of the model run. The jump to Sigma II was made that much easier because of what Cadillac had learned from the CTS-V.
I think what you need to get your head around here is that first, you are buying a Cadillac. With that, despite the common parts shared with other GM vehicles, you are going to have to pay a lot more to fix a "normal" issue than you would with similar choices offered at the time. Similarly, the comparative rarity of the LS6 could bring further issues later on down the road. That isn't saying that the engine wouldn't be bulletproof, but considering that it was only available in the Corvette and CTS-V, it isn't as though the bits and pieces are as common as what was later offered by the LS2 and LS3.
Confounding these issues, at least for me, are the fact that two cars from Pontiac showed up in roughly the same window as the Sigma CTS-V. First was the GTO, which arguably had better build quality and similar performance, particularly with the later LS2-powered versions. Second, the G8 GT (and G8 GXP) created even more issues for the Sigma CTS-V, especially in terms of practicality. Although the G8 lacked some of the muscle compared to the CTS-V, it was still a very capable sport sedan, and was certainly the last hurrah for a dying company.
I would be hard-pressed to choose the Sigma CTS-V over the GTO, if I am being honest. Dollar-for-dollar, the Pontiac is a better deal if you are capable of sacrificing one seat in the back and two doors. That is not to say that I do not love the Sigma CTS-V, but more or less that I would not be willing to take the dip in the Cadillac pricing pool. They are excellent vehicles, there is no way to question that, but if you are looking for something that is going to be inherently reliable and reasonable to repair, the CTS-V is not the car of choice.
TL;DR - The CTS-V is great if you can afford it. Otherwise, get a GTO.
* References: Being the "GM Guy" around here