I owned a 2001. I'll give you my every thought on it.
First, it's perhaps the nicest car I've ever owned, in terms of feature set and over all cache. You feel like a million bucks driving it. It's pretty fast, and okay on fuel. Although regular fuel is recommended, premium wakes the engine up pretty good.
I couldn't speak to reliability, because I think mine was inordinarily bad. I found out after I bought it that the whole car had nearly been rebuilt. It was an Ohio car, but I don't think that gives a good reason for all the stuff that had been replaced (rust, etc). The entire exhaust system had been replaced. The radio had been replaced. The instrument cluster had been replaced. One of the seats had been replaced due to no heat. All of the pistons had been replaced due to oil consumption, and 10k miles later, GM replaced the entire engine (I guess the piston/ring kit didn't help). The replacement engine had a knock to it (piston slap probably) that I never really liked. Shortly after I bought it, I had to have the intermediate steering shaft replaced (very common for GMs of that era). I had it to the dealer 4 times for that, and it was mis-diagnosed as a number of things before they actually fixed the problem. They replaced both $900 front struts to no avail. Then they messed up the strut mount and had to replace one of those. These G-chassis cars are also extremely prone to front wheel vibrations. They came out with a revised lower control arm to help quell that, at least for some of the Pontiacs or Buicks (with a large hydro bushing). I'm not sure if the Seville has a revised LCA. I fought wheel balance issues with mine, and a road force balance fixed most of it. I still wasn't completely satisfied.
As far as the engine in general, they can consume oil. Not all do, and some consume oil pretty heavily. It's a luck of the draw thing on that. And the Northstar had gone through a few revisions, and the 2000+ redesign largely fixed the head gasket problem, or at least reduced the rate of occurance. Another tweak in 2004 largely cured it, but the Seville never benefitted from that. If he were buying one, I strongly encourage a 2000+ model for this reason. Annual coolant changes are not necessary. These cars run fine with the Dexcool. Head gasket problems are a function of the mechanical strength of the head bolt threads in the block, not the coolant.
As far as the driving, it was a mixed bag. Cadillac liked to compare it to a BMW, but I don't think it drove all that well personally. It is a large car, and it handles pretty well for a large car. But it wasn't all that satisfying to drive. The front suspension feels overly stiff to compensate for it being FWD, while still looking for some handling prowess. The Magnasteer rack wasn't very smooth...you could feel differences in effort during fast sweeping turns that was disconcerting. It wasn't a very balanced package to me. If it says anything, the Corolla I traded the STS for felt like a much more balanced and organic package. The STS feels soul-less, just like the Camry did that I used to have. The STS very much feels like a digital machine...it's competent, but you don't get that it-feels-like-an-extension-of-my-hands feeling that you do with better driving cars. I told a co-worker after my trade that I had more fun driving my Corolla at 4 10ths than I had driving the STS at 8 10ths. You could make the STS dance, but it wasn't all that much fun to do so.
Mine was White Diamond, and it was a stunningly beautiful car. It just didn't "click" with me like my '97 did. Here are some pictures of both of them together. I sold the '97 to my brother shortly after I bought the '01. Never should have bought the '01.