I have 2, and have played 3 others. I have owned 2 Alembics, a Tobias, a Mouradian, and some high quality basses from larger companies such as Rickenbacker, Warwick, and the old US BC Rich. I have played basses in stores by companies such as MTD, PRS, Status, Lakland, and Sadowsky. Never seen a Fodera, Fbass, or most of the other names you mostly only hear on Talkbass (not a slam, but people here are often more aware than the average musician of these things).
Mine are both early models; his 3rd fretless, and the 1st Professional Series bass built. First the similarities in all the Ristolas I've played:
They all sound, play, and look great, easily on par with any boutique bass I have experienced. Brian uses a variety of woods and electronics, so the character of each instrument is different. Some are warm and vintage sounding; I played one with a Demeter preamp that had a super clean, hi fi sound. Not my sound at all, but it sounded awesome. The attention to detail in general seems to be very impressive. On the website you can see that whether you like spectacularly topped, flamboyant basses or simpler, painted "classic" looking basses there's something for you. His necks have the perfect feel for me. My 5 string is as comfortable as many 4 strings, and the fretless has a neck that is a joy to play.
The pickup question is out of my league, it would depend on the particular bass; it might be possible to get specs from the manufacturers' websites. If you are thinking about a custom order, I would imagine you could get any pickup you wanted. I don't know what the lead time is, or even if he is taking orders ATM, but unlike some luthiers he actually answers emails.
There are things about mine that I would have changed if they were custom orders, but I can't complain since they weren't. I would never have thought to order a bolt on, painted 5 string, but I played it out of curiosity and the sound was PERFECT for my needs. He knows what he's doing, that's for sure. The only actual negative quibble I could have would be that the batteries in the 5 string went dead quickly. I could have had Brian take a look at it under warranty, but I didn't want to be without the bass so I fixed it myself.
Before my career imploded, I had planned to have a custom made. Perhaps one day I will be able to afford it. I do worry though that if I did that I would never again play my Alembic.
the 5 sounds like this (well, one of the sounds)
http://www.talkbass.com/forum/showpost.php?p=3330729&postcount=35
The fretless looks like this (no clips)
http://www.talkbass.com/forum/showpost.php?p=2952693&postcount=8