I have owned many Carvins, including an SB500o and they were great basses, especially for the money. IMHO, the B25 'Bromberg' is the best they make, I miss my fretted one but still have a fretless B25. Nowadays, with the whole 'Kiesel' thing, prices have gone up in most cases, and in some cases a laughable amount. My Carvins are/were fanstastic basses at the $1k-$1200 range.
Regarding comparing them, IME a basic, non-pimped Carvin version it will hold up well against US Fenders and other basses within a few hundred dollars it's price range. But when you start getting into coffee table tops and some of the pimps-on-acid finishes, you can suddenly be looking at a bass in the $2K-$4k range and that brings a whole lot of other big-time basses into play (Sadowsky, Wilkins, Lull, Valenti, Nordstrand and others). I've owned some of those boutiques and since they either offer or ship with top-of-the-line, time-tested and highly desired electronics, I would have to give a nod to them for sound. IMHO, $3K Kiesels are not going to sound or play any better than the $1K Carvins (same hardware, pickups and electronics), but if cosmetics are your main thing, then they offer a lot of choices if you are willing to pay for it (and potentially lose your a$$ if you ever decide to sell it....).