I have three 519's, 2 fretted and 1 fretless and won't give them up for nothing, I had a 4 string, but Simon now owns it. (Long story) The 519 he has used to be my friends, I almost bought it but Simon beat me to it, so I wound up with another 519 with a similar spalted top. That's the bass that started it all, then I ordered the 4 string that Simon has, I played that for a while then traded it towards a Koa 519 and just picked up a fretless 519 a few weeks ago.
I play every weekend and live in Georgia, the weather changes quite a bit, I never have to worry about adjusting the neck or anything, it's always the same. The Bart jazz pickups cut through like butter, tone is killer, the action can go as low as you want, and it balances well.
If you can get one, do it, they are worth it. I've played some really good stuff over the last year, but hands down for the money, I don't think you can beat the Zon's.
Mike