Ok ... here are some pictures (they're off the Sei site but I may take some of my own later) so my bass now officially exists.
I'm over the initial extra string awkwardness but I haven't quite gotten used to not having the the quick geographical reference of two dots on the fingerboard at the 12th fret. Having said that I have no regrets about the blocks and for insisting on the size. They created a bit of a problem in construction as Martin was concerned about the ability to get enough camber on them to match the neck. In the end he did a great job with them and I love 'em.
My main bass for the past 6 years or so has been a Sadowsky Metro MV4. It's a fantastic instrument that records with clarity and cuts well live, however in my heart of hearts it's a bit too modern sounding in some situations. With that in mind the tonal aim for the Sei build was a classic warm Jazz bass sound with some help from the EQ when needed.
Tonally...well...it's a Jazz bass, mission accomplished. In pure passive mode with pickups and tone wide open the acoustic warmth of the note opens up with a bit more texture in the mids and a lovely crisp top end. Based on a couple of days in the living room and one rehearsal I'm very impressed with the Nordstrand SV pickups. I have a set of the SEs in a Fender Jazz (can you tell I like Js at all?

) and spent alot of time humming and hawwing about these vs the SVs for the build.
The SVs have a nicer midrange colour to me, particularly with both wide open. The bridge pickup is thinner but actually I don't find that I solo it much in a band context so it doesn't feel much of a downside.
The Nordstand circuit does colour the sound a little even when set flat but you'd be doing well to notice once the band is blasting away. The impact of the cut and boost seems rather more subtle than the Sadowsky circuit. This is either a positive or negative depending on taste. In terms of this build I think it's a positive as the passive tone of the instrument is a bit more intact.
There is always a bit of risk involved when you go for a custom build as you don't really know what you're getting until it arrives. Sitting here now after my first weekend with a bass I ordered on 1 December 2010 I feel like I've got what I had in mind both visually and sonically.