To help you make an assessment before you make a purchase, the XLR advertised is nice, but average. And way overpriced - though negotiable. I bought a similar car, with less mileage seven years ago for $32k. A dealer would offer < $15k for a 2004, which is still about $10k more than any other 11 year-old Cadillac. The good news is XLR's have retained their value better than any Cadillac in the last thirty years. Compared to the similarly-priced ELR, the XLR will most likely retain the best depreciation title, since the ELR has sold about half the 2014 inventory and will skip the 2015 model year in order to clear excess inventory and instead get an upgrade for the 2016 year. (Not much of a sales incentive, with an improved model coming out next year, but the sticker price is down 30%, much to the chagrin of early adopters.)
There was no XLR marketed as a "Bulgari Edition", but people can call their cars whatever they want when selling. The Bulgari logo placed over the speedometer and tachometer was removed late in production, but the basic instrumentation design (done by Bulgari) remained throughout the XLRs all-too-short lifespan.
So to answer your questions, this car while nice, isn't rare, (2004-2005 was nearly half of total XLR production.) It's asking price is overpriced, (but negotiable) and future value is better than average, though I wouldn't consider it an investment until another twenty years passes. For additional research questions, this is a great place to pose questions.
Some XLR-specific parts are getting hard to find, the cars are aging and more prone to failure, so they need to be really taken care of to retain their value. 31 MPG is nice, I've only hit that (once) during long freeway road-trips.As for value, XLR is a bargain for the 2004-2006 model years as long as the price is right. They are able cars with plenty of power and just as attractive as any newer model Cadillac and never fail to start a conversation at a gas pump, as they are still rarely seen in the wild. (I saw my first XLR in over a year the other night.) I
If you can score a nice 2004 for <$15k, go for it. Kelly Blue Book prices an excellent condition 2004 XLR at about $12K for a private party seller (depending on zip code.) A dealer will ask for more.
CC