I've had a few Sei basses and a Shuker bass, most custom-built to my specification.
Both Martin Petersen and Jon Shuker are absolutely great guys to deal with and both had plenty of time to listen to my thoughts & suggestions.
Like Krysh said, the Sei basses are almost like furniture with attention to detail. However, that comes with it's own problems and I actually found them almost too delicate for the rigours of gigging having to continually tweak truss rods and adjust the neck on more than one of the basses. My last bass took 18 months to complete and in my opinion that's too long. I was quoted 9 months. I'll be honest, the increase in costs by Sei for their Jazz basses now mean they are too high for me and I looked at another builder.
The Shuker is now my workhorse bass and plays like an absolute dream. Jon Shuker was happy to do anything for me. I wanted a sparkle finish, which he was happy to do. He even went to the extent of re-shaping the neck and swapping out the inlay blocks on the neck because I wasn't happy. This was after I'd received the finished bass!!! This was all done free of charge (sorry Jon, I shouldn't say that) because he wanted me to be 100% happy with my bass. Build took around 9 months, when I was quoted 6 months and was about 1/2 the price of the Sei. An absolute bargain for the money.
At the end of the day though, I would suggest you get a good idea of what you want and then speak to a number of builders. You'll soon know who you'd like to work with. Remember though, it's addictive !!!!