I've had three. From Cheapest to most expensive:
[b]Wish Bass[/b]
Pretty much told Wish I wanted a stripy peanut with a purpleheart board, one pickup & a truss rod.
Didn't go exactly to plan. Two pups, no truss-rod. Ended up replacing the bridge, finishing it in oil. Cost: $700. Would I buy anther one: No. Result: Sold on e-bay for £250.
[b]Carvin[/b]
It's difficult to buy a Carvin without it being a custom. Filled out a form on their web-site detailing the options.
Didn't go exactly to plan; the neck ended up with a gloss rather than matt finish. No modifications. Cost: $1,400. Would I buy anther one: Possibly. Result: swapped on basschat for a fretless Warwick.
[b]GW[/b]
Probably spent the longest time speccing (and waiting) for this bass. Went exactly to plan. Grandon built me exactly what I asked for. Sadly it turns out I don't really know what I was after. Cost: $2,300 (including import duty to this country). Would I buy another: No. Result: Sits on my wall looking pretty.
Draw your own conclusions.
Is it worth it? IMO - Not really. As Davo says you have to really know what you want. Which I don't. My days of one off customs are almost certainly over.
TBH, if you buy an up-scale off the shelf bass, second hand on here, you don't loose that much money if you decide to sell them on.
Having said that it [i]is[/i] enourmous fun.